HTTP interface for clusters

The cluster-specific endpoints let you get information about individual cluster nodes and the cluster as a whole, as well as monitor and administrate cluster deployments

Monitoring

Get the statistics of a DB-Server

get /_admin/cluster/statistics

Queries the statistics of the given DB-Server

Path Parameters

Query Parameters

DBserver* string
The ID of a DB-Server.

HTTP Headers

Responses

200
is returned when everything went well.
400
The DBserver parameter was not specified or is not the ID of a DB-Server.
403
The specified server is not a DB-Server.

Get the cluster health

get /_admin/cluster/health

Queries the health of the cluster as assessed by the supervision (Agency) for monitoring purposes. The response is a JSON object, containing the standard code, error, errorNum, and errorMessage fields as appropriate. The endpoint-specific fields are as follows:

ClusterId: A UUID string identifying the cluster
Health: An object containing a descriptive sub-object for each node in the cluster.
- <nodeID>: Each entry in Health will be keyed by the node ID and contain the following attributes:
  - Endpoint: A string representing the network endpoint of the server.
  - Role: The role the server plays. Possible values are "AGENT", "COORDINATOR", and "DBSERVER".
  - CanBeDeleted: Boolean representing whether the node can safely be removed from the cluster.
  - Version: Version String of ArangoDB used by that node.
  - Engine: Storage Engine used by that node.
  - Status: A string indicating the health of the node as assessed by the supervision (Agency). This should be considered primary source of truth for Coordinator and DB-Servers node health. If the node is responding normally to requests, it is "GOOD". If it has missed one heartbeat, it is "BAD". If it has been declared failed by the supervision, which occurs after missing heartbeats for about 15 seconds, it will be marked "FAILED".
  Additionally it will also have the following attributes for:
  Coordinators and DB-Servers
  - SyncStatus: The last sync status reported by the node. This value is primarily used to determine the value of Status. Possible values include "UNKNOWN", "UNDEFINED", "STARTUP", "STOPPING", "STOPPED", "SERVING", "SHUTDOWN".
  - LastAckedTime: ISO 8601 timestamp specifying the last heartbeat received.
  - ShortName: A string representing the shortname of the server, e.g. "Coordinator0001".
  - Timestamp: ISO 8601 timestamp specifying the last heartbeat received. (deprecated)
  - Host: An optional string, specifying the host machine if known.
  Coordinators only
  - AdvertisedEndpoint: A string representing the advertised endpoint, if set. (e.g. external IP address or load balancer, optional)
  Agents
  - Leader: ID of the Agent this node regards as leader.
  - Leading: Whether this Agent is the leader (true) or not (false).
  - LastAckedTime: Time since last acked in seconds.

Responses

200
is returned when everything went well.

Endpoints

List all Coordinator endpoints

get /_api/cluster/endpoints

Returns an object with an attribute endpoints, which contains an array of objects, which each have the attribute endpoint, whose value is a string with the endpoint description. There is an entry for each Coordinator in the cluster. This method only works on Coordinators in cluster mode. In case of an error the error attribute is set to true.

Responses

200
is returned when everything went well.
- code* integer
  the HTTP status code - 200
- endpoints* array of objects
  A list of active cluster endpoints.
  endpoint* string
  The bind of the Coordinator, like tcp://[::1]:8530
- error* boolean
  boolean flag to indicate whether an error occurred (true in this case)
501
server is not a Coordinator or method was not GET.

Cluster node information

Get the server ID

get /_admin/server/id

Returns the ID of a server in a cluster. The request will fail if the server is not running in cluster mode.

Responses

200
Is returned when the server is running in cluster mode.
500
Is returned when the server is not running in cluster mode.

Get the server role

get /_admin/server/role

Returns the role of a server in a cluster. The server role is returned in the role attribute of the result.

Responses

200
Is returned in all cases.
- code* integer
  the HTTP status code, always 200
- error* boolean
  always false
- errorNum* integer
  the server error number
- role* string
  The server role. Possible values:
  SINGLE: the server is a standalone server without clustering
  COORDINATOR: the server is a Coordinator in a cluster
  PRIMARY: the server is a DB-Server in a cluster
  SECONDARY: this role is not used anymore
  AGENT: the server is an Agency node in a cluster
  UNDEFINED: in a cluster, this is returned if the server role cannot be determined.

Maintenance

Set the cluster maintenance mode

put /_admin/cluster/maintenance

Enable or disable the cluster supervision (Agency) maintenance mode.

This API allows to temporarily enable the supervision maintenance mode. Please be aware that no automatic failovers of any kind will take place while the maintenance mode is enabled. The cluster supervision reactivates itself automatically at some point after disabling it.

To enable the maintenance mode the request body must contain the string "on" (Please note it must be lowercase as well as include the quotes). This will enable the maintenance mode for 60 minutes, i.e. the supervision maintenance will reactivate itself after 60 minutes.

Since ArangoDB 3.8.3 it is possible to enable the maintenance mode for a different duration than 60 minutes, it is possible to send the desired duration value (in seconds) as a string in the request body. For example, sending "7200" (including the quotes) will enable the maintenance mode for 7200 seconds, i.e. 2 hours.

To disable the maintenance mode the request body must contain the string "off" (Please note it must be lowercase as well as include the quotes).

Responses

200
is returned when everything went well.
400
if the request contained an invalid body
501
if the request was sent to a node other than a Coordinator or single-server
504
if the request timed out while enabling the maintenance mode

Get the maintenance status of a DB-Server

get /_admin/cluster/maintenance/{DB-Server-ID}

Check whether the specified DB-Server is in maintenance mode and until when.

Path Parameters

DB-Server-ID* string
The ID of a DB-Server.

Query Parameters

HTTP Headers

Responses

200
The request was successful.
- code* integer
  The status code. 200 in this case.
- error* boolean
  Whether an error occurred. false in this case.
- result object
  The result object with the status. This attribute is omitted if the DB-Server is in normal mode.
  Mode string
  The mode of the DB-Server. The value is "maintenance".
  Until string
  Until what date and time the maintenance mode currently lasts, in the ISO 8601 date/time format.
400
if the request contained an invalid body
412
if the request was sent to an Agent node
504
if the request timed out while enabling the maintenance mode

Set the maintenance status of a DB-Server

put /_admin/cluster/maintenance/{DB-Server-ID}

Enable or disable the maintenance mode of a DB-Server.

For rolling upgrades or rolling restarts, DB-Servers can be put into maintenance mode, so that no attempts are made to re-distribute the data in a cluster for such planned events. DB-Servers in maintenance mode are not considered viable failover targets because they are likely restarted soon.

Path Parameters

DB-Server-ID* string
The ID of a DB-Server.

Query Parameters

HTTP Headers

Request Body application/json

mode* string
The mode to put the DB-Server in. Possible values:
- "maintenance"
- "normal"
timeout integer
After how many seconds the maintenance mode shall automatically end. You can send another request when the DB-Server is already in maintenance mode to extend the timeout.

Responses

200
The request was successful.
- code* integer
  The status code. 200 in this case.
- error* boolean
  Whether an error occurred. false in this case.
400
if the request contained an invalid body
412
if the request was sent to an Agency node
504
if the request timed out while enabling the maintenance mode

Rebalance

As of version 3.10, ArangoDB has built-in capabilities to rebalance the distribution of shards. This might become necessary since imbalances can lead to uneven disk usage across the DBServers (data unbalance) or to uneven load distribution across the DBServers (leader imbalance).

If the data is distributed relatively evenly across the DBServers, then the leader imbalance can usually be adjusted relatively cheaply, since you only have to transfer leadership for a number of shards to a different replica, which already has the data. This is not true in all cases, but as a rule of thumb it is true.

If, however, data needs to be moved between DBServers, then this is a costly and potentially lengthy operation. This is inevitable, but it has been made in this way so that this operation is done in the background and does not lead to service interruption.

Nevertheless, data movement requires I/O, CPU, and network resources and thus it always puts an additional load on the cluster.

Rebalancing shards is a rather complex optimization problem, in particular if there are many shards in the system. Fortunately, in most situations it is relatively easy to find operations to make good progress towards a better state, but “perfection” is hard, and finding a “cheap” way to get there is even harder.

The APIs described here try to help with the following approach: There is an “imbalance score” which is computed on a given shard distribution, which basically says how “imbalanced” the cluster is. This score involves leader imbalance as well as data imbalance. Higher score means more imbalance, the actual numerical value does not have any meaning.

The GET API call can be used to evaluate this score and give back how imbalanced the cluster currently is. The POST API call does the same and additionally computes a list of shard movements which the system suggests to lower the imbalance score. A variant of the POST API call can then take this (or another) suggestion and execute it in the background. For convenience, you can use the PUT API call to do all at once: compute the score, suggest moves, and execute them right away. Since the execution can take some time, the GET API call also tells you how many of the moves are still outstanding.

There are three types of moves:

Switch leadership of one shard from the leader to a follower, which is currently in sync. This is a fast move.
Move the data of a leader to a new DBServer and make it the new leader. This is a slow move, since it needs to copy the data over the network and then switch the leadership.
Move the data of a follower to a new DBServer and make it a new follower, then drop the data on the previous follower. This is a slow move, since it needs to copy the data over the network.

The suggestion engine behind the POST and PUT API calls has three switches to activate/deactivate these three types of moves independently. If a type of move is activated, the engine considers all possible such moves, if it is deactivated, no such moves are considered. The three flags are:

leaderChanges (default true): consider moves of type 1.
moveLeaders (default false): consider moves of type 2.
moveFollowers (default false): consider moves of type 3.

The engine then enumerates all possible moves of the various types. The first choice is the one which improves the imbalance the most. After that move, it reevaluates the imbalance score and again look for the move which improves the imbalance score the most. It altogether suggests up to maximumNumberOfMoves moves and then stops. The default value for this maximum is 1000 and it is capped at 5000 to avoid overly long optimization computations. It is conceivable that for large clusters, 1000 or even 5000 might not be enough to achieve a full balancing. In such cases, you simply have to repeat the API calls potentially multiple times.

Other considerations

First, in the case of smart graphs or one shard databases, not all shards can be moved freely. Rather, some shards are “coupled” and can only move their place in the cluster or even their leadership together. This severely limits the possibilities of shard movement and sometimes makes a good balance impossible. A prominent example here is a single one shard database in the cluster. In this case, all leaders have to reside on the same server, so no good leader distribution is possible at all.

Secondly, the current implementation does not take actual shard sizes into account. It essentially works on the number of shards and tries to distribute the numbers evenly. It computes weights on the grounds of how many shards are “coupled” together, but it does not take actual data size into account. This means that it is possible that we get a “good” data distribution w.r.t. number of shards, but not with respect to their disk size usage.

Thirdly, the current implementation does not take compute load on different collections and shards into account. Therefore, it is possible that we end up with a shard distribution which distributes the leader numbers evenly across the cluster, even though the actual compute load is then unevenly distributed, since some collections/shard simply are hit by more queries than others.

How to use the rebalancing API

By far, the easiest way to rebalance a cluster is to simply call the PUT variant of the API, which analyzes the situation, comes up with a plan to balance things out, and directly schedules it. To rebalance leaders, you can use curl like this:

curl -X PUT https://<endpoint>:<port>/_admin/cluster/rebalance -d '{"version":1}' --user root:<rootpassword>

You need admin rights, so you should use the user root or another user with write permissions on the _system database. Alternatively, you can use a header with a valid JWT token (for superuser access).

Note that this API call only triggers the rebalancing operation, the API call returns before the actual rebalancing is finished!

Since the default value for leaderChanges is true and for moveLeaders and moveFollowers is false, this only schedules cheap leader changes. So it can address leader imbalance, but not data imbalance.

You can monitor progress with this command:

curl https://<endpoint>:<port>/_admin/cluster/rebalance --user root:<rootpassword>

The resulting object looks roughly like this:

{
  "error": false,
  "code": 200,
  "result": {
    "leader": {
      "weightUsed": [
        51,
        54,
        53
      ],
      "targetWeight": [
        52.666666666666664,
        52.666666666666664,
        52.666666666666664
      ],
      "numberShards": [
        31,
        54,
        21
      ],
      ...
      "imbalance": 1920000004.6666667
    },
    "shards": {
      "sizeUsed": [
        60817408,
        106954752,
        54525952
      ],
      "targetSize": [
        74099370.66666666,
        74099370.66666666,
        74099370.66666666
      ],
      "numberShards": [
        58,
        102,
        52
      ],
      ...
      "imbalance": 1639005333138090.8
    },
    "pendingMoveShards": 0,
    "todoMoveShards": 0
  }
}

Of particular relevance are the two attributes pendingMoveShards and todoMoveShards. These show how many move operations are still to do (scheduled, but not begun), and how many are pending (scheduled, started, but not yet finished). Once these two numbers have reached 0, the rebalancing operation is finished.

In the leader section you see stats about the distribution of the leader shards, in the shards section you see stats about the distribution of the data (leader shards and follower shards). In both sections, we see numbers for numberShards and for the current distribution (weightUsed for leaders and sizeUsed for shards), as well as for the target distribution. Finally, the imbalance number is the “imbalance score”, its absolute value is not meaningful, but the smaller the score, the better the balance is.

If you actually want to allow the system to move data to improve the data distribution, use this command:

curl -X PUT https://<endpoint>:<port>/_admin/cluster/rebalance -d '{"version":1, "moveLeaders": true, "moveFollowers": true}' --user root:<rootpassword>

Note that this API call only triggers the rebalancing operation, the API call returns before the actual rebalancing is finished!

This operation is monitored with the same GET request as above, it is expected that it takes considerably longer to finish.

There are a few more knobs to turn in this, but these should usually not be necessary and are intended only for expert use.

Note that both these API calls only ever schedules up to 1000 move shard jobs. For large data sets, you might want to repeat the call after completion.

Get the current cluster imbalance

get /_admin/cluster/rebalance

Computes the current cluster imbalance and returns the result. It additionally shows the amount of ongoing and pending move shard operations.

Responses

200
This API returns HTTP 200.
- code* number
  The status code.
- error* boolean
  Whether an error occurred. false in this case.
- pendingMoveShards* number
  The number of pending move shard operations.
- result* object
  The result object.
  leader* object
  Information about the leader imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  leaderDupl* array of integers
  The measure of the leader shard distribution. The higher the number, the worse the distribution.
  numberShards* array of integers
  The number of leader shards per DB-Server.
  targetWeight* array of integers
  The ideal weight of leader shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader shards only.
  totalWeight* integer
  The sum of all weights.
  weightUsed* array of integers
  The weight of leader shards per DB-Server. A leader has a weight of 1 by default but it is higher if collections can only be moved together because of distributeShardsLike.
  shards* object
  Information about the shard imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  numberShards* array of integers
  The number of leader and follower shards per DB-Server.
  sizeUsed* array of integers
  The size of shards per DB-Server.
  targetSize* array of integers
  The ideal size of shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader and follower shards.
  totalShardsFromSystemCollections* integer
  The sum of system collection shards, counting leader shards only.
  totalUsed* integer
  The sum of the sizes.
- todoMoveShards* number
  The number of planned move shard operations.

Compute a set of move shard operations to improve balance

post /_admin/cluster/rebalance

Compute a set of move shard operations to improve balance.

Request Body application/json

databasesExcluded array of strings
A list of database names to exclude from the analysis. (Default: [])
excludeSystemCollections boolean
Ignore system collections in the rebalance plan. (Default: false)
leaderChanges boolean
Allow leader changes without moving data. (Default: true)
maximumNumberOfMoves number
Maximum number of moves to be computed. (Default: 1000)
moveFollowers boolean
Allow moving followers. (Default: false)
moveLeaders boolean
Allow moving leaders. (Default: false)
piFactor number
A weighting factor that should remain untouched. (Default: 256e6)
If a collection has more shards than there are DB-Servers, there can be a subtle form of leader imbalance. Some DB-Servers may be responsible for more shards as leader than others. The piFactor adjusts how much weight such imbalances get in the overall imbalance score.
version* number
Must be set to 1.

Responses

200
This API returns HTTP 200.
- code* number
  The status code.
- error* boolean
  Whether an error occurred. false in this case.
- result* object
  The result object.
  imbalanceAfter* object
  Expected imbalance after the suggested move shard operations are applied.
  leader* object
  Information about the leader imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  leaderDupl* array of integers
  The measure of the leader shard distribution. The higher the number, the worse the distribution.
  numberShards* array of integers
  The number of leader shards per DB-Server.
  targetWeight* array of integers
  The ideal weight of leader shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader shards only.
  totalWeight* integer
  The sum of all weights.
  weightUsed* array of integers
  The weight of leader shards per DB-Server. A leader has a weight of 1 by default but it is higher if collections can only be moved together because of distributeShardsLike.
  shards* object
  Information about the shard imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  numberShards* array of integers
  The number of leader and follower shards per DB-Server.
  sizeUsed* array of integers
  The size of shards per DB-Server.
  targetSize* array of integers
  The ideal size of shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader and follower shards.
  totalShardsFromSystemCollections* integer
  The sum of system collection shards, counting leader shards only.
  totalUsed* integer
  The sum of the sizes.
  imbalanceBefore* object
  Imbalance before the suggested move shard operations are applied.
  leader* object
  Information about the leader imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  leaderDupl* array of integers
  The measure of the leader shard distribution. The higher the number, the worse the distribution.
  numberShards* array of integers
  The number of leader shards per DB-Server.
  targetWeight* array of integers
  The ideal weight of leader shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader shards only.
  totalWeight* integer
  The sum of all weights.
  weightUsed* array of integers
  The weight of leader shards per DB-Server. A leader has a weight of 1 by default but it is higher if collections can only be moved together because of distributeShardsLike.
  shards* object
  Information about the shard imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  numberShards* array of integers
  The number of leader and follower shards per DB-Server.
  sizeUsed* array of integers
  The size of shards per DB-Server.
  targetSize* array of integers
  The ideal size of shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader and follower shards.
  totalShardsFromSystemCollections* integer
  The sum of system collection shards, counting leader shards only.
  totalUsed* integer
  The sum of the sizes.
  moves* array of objects
  The suggested move shard operations.
  collection* number
  Collection ID of the collection the shard belongs to.
  from* string
  The server name from which to move.
  isLeader* boolean
  True if this is a leader move shard operation.
  shard* string
  Shard ID of the shard to be moved.
  to* string
  The ID of the destination server.

Execute a set of move shard operations

post /_admin/cluster/rebalance/execute

Execute the given set of move shard operations. You can use the POST /_admin/cluster/rebalance endpoint to calculate these operations to improve the balance of shards, leader shards, and follower shards.

Request Body application/json

moves* array of objects
A set of move shard operations to execute.
- collection* number
  Collection ID of the collection the shard belongs to.
- from* string
  The server name from which to move.
- isLeader* boolean
  True if this is a leader move shard operation.
- shard* string
  Shard ID of the shard to be moved.
- to* string
  The ID of the destination server.
version* number
Must be set to 1.

Responses

200
This API returns HTTP 200 if no operations are provided.
202
This API returns HTTP 202 if the operations have been accepted and scheduled for execution.

Compute and execute a set of move shard operations to improve balance

put /_admin/cluster/rebalance

Compute a set of move shard operations to improve balance. These moves are then immediately executed.

Request Body application/json

databasesExcluded array of strings
A list of database names to exclude from the analysis. (Default: [])
excludeSystemCollections boolean
Ignore system collections in the rebalance plan. (Default: false)
leaderChanges boolean
Allow leader changes without moving data. (Default: true)
maximumNumberOfMoves number
Maximum number of moves to be computed. (Default: 1000)
moveFollowers boolean
Allow moving followers. (Default: false)
moveLeaders boolean
Allow moving leaders. (Default: false)
piFactor number
A weighting factor that should remain untouched. (Default: 256e6)
If a collection has more shards than there are DB-Servers, there can be a subtle form of leader imbalance. Some DB-Servers may be responsible for more shards as leader than others. The piFactor adjusts how much weight such imbalances get in the overall imbalance score.
version* number
Must be set to 1.

Responses

200
This API returns HTTP 200.
- code* number
  The status code.
- error* boolean
  Whether an error occurred. false in this case.
- result* object
  The result object.
  imbalanceAfter* object
  Expected imbalance after the suggested move shard operations are applied.
  leader* object
  Information about the leader imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  leaderDupl* array of integers
  The measure of the leader shard distribution. The higher the number, the worse the distribution.
  numberShards* array of integers
  The number of leader shards per DB-Server.
  targetWeight* array of integers
  The ideal weight of leader shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader shards only.
  totalWeight* integer
  The sum of all weights.
  weightUsed* array of integers
  The weight of leader shards per DB-Server. A leader has a weight of 1 by default but it is higher if collections can only be moved together because of distributeShardsLike.
  shards* object
  Information about the shard imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  numberShards* array of integers
  The number of leader and follower shards per DB-Server.
  sizeUsed* array of integers
  The size of shards per DB-Server.
  targetSize* array of integers
  The ideal size of shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader and follower shards.
  totalShardsFromSystemCollections* integer
  The sum of system collection shards, counting leader shards only.
  totalUsed* integer
  The sum of the sizes.
  imbalanceBefore* object
  Imbalance before the suggested move shard operations are applied.
  leader* object
  Information about the leader imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  leaderDupl* array of integers
  The measure of the leader shard distribution. The higher the number, the worse the distribution.
  numberShards* array of integers
  The number of leader shards per DB-Server.
  targetWeight* array of integers
  The ideal weight of leader shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader shards only.
  totalWeight* integer
  The sum of all weights.
  weightUsed* array of integers
  The weight of leader shards per DB-Server. A leader has a weight of 1 by default but it is higher if collections can only be moved together because of distributeShardsLike.
  shards* object
  Information about the shard imbalance.
  imbalance* integer
  The measure of the total imbalance. A high value indicates a high imbalance.
  numberShards* array of integers
  The number of leader and follower shards per DB-Server.
  sizeUsed* array of integers
  The size of shards per DB-Server.
  targetSize* array of integers
  The ideal size of shards per DB-Server.
  totalShards* integer
  The sum of shards, counting leader and follower shards.
  totalShardsFromSystemCollections* integer
  The sum of system collection shards, counting leader shards only.
  totalUsed* integer
  The sum of the sizes.
  moves* array of objects
  The suggested move shard operations.
  collection* number
  Collection ID of the collection the shard belongs to.
  from* string
  The server name from which to move.
  isLeader* boolean
  True if this is a leader move shard operation.
  shard* string
  Shard ID of the shard to be moved.
  to* string
  The ID of the destination server.