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More about Dgraph Zero

Dgraph Zero controls the Dgraph cluster, and stores information about it. It automatically moves data between different Dgraph Alpha instances based on the size of the data served by each Alpha instance.

Before you can run dgraph alpha, you must run at least one dgraph zero node. You can see the options available for dgraph zero by using the following command:

dgraph zero --help

The --replicas option controls the replication factor: the number of replicas per data shard, including the original shard. For consensus, the replication factor must be set to an odd number, and the following error will occur if it is set to an even number (for example, 2):

ERROR: Number of replicas must be odd for consensus. Found: 2

When a new Alpha joins the cluster, it is assigned to a group based on the replication factor. If the replication factor is set to 1, then each Alpha node will serve a different group. If the replication factor is set to 3 and you then launch six Alpha nodes, the first three Alpha nodes will serve group 1 and next three nodes will serve group 2. Zero monitors the space occupied by predicates in each group and moves predicates between groups as-needed to rebalance the cluster.

Endpoints

Like Alpha, Zero also exposes HTTP on port 6080 (plus any ports specified by --port_offset). You can query this port using a GET request to access the following endpoints:

  • /state returns information about the nodes that are part of the cluster. This includes information about the size of predicates and which groups they belong to.
  • /assign?what=uids&num=100 allocates a range of UIDs specified by the num argument, and returns a JSON map containing the startId and endId that defines the range of UIDs (inclusive). This UID range can be safely assigned externally to new nodes during data ingestion.
  • /assign?what=timestamps&num=100 requests timestamps from Zero. This is useful to “fast forward” the state of the Zero node when starting from a postings directory that already has commits higher than Zero’s leased timestamp.
  • /removeNode?id=3&group=2 removes a dead Zero or Alpha node. When a replica node goes offline and can’t be recovered, you can remove it and add a new node to th quorum. To remove dead Zero nodes, pass group=0 and the id of the Zero node to this endpoint.
Note Before using the API ensure that the node is down and ensure that it doesn’t come back up ever again. Do not use the same idx of a node that was removed earlier.
  • /moveTablet?tablet=name&group=2 Moves a tablet to a group. Zero already rebalances shards every 8 mins, but this endpoint can be used to force move a tablet.

You can also use the following POST endpoint on HTTP port 6080:

  • /enterpriseLicense applies an enterprise license to the cluster by supplying it as part of the body.

More About the /state Endpoint

The /state endpoint of Dgraph Zero returns a JSON document of the current group membership info, which includes the following:

  • Instances which are part of the cluster.
  • Number of instances in Zero group and each Alpha groups.
  • Current leader of each group.
  • Predicates that belong to a group.
  • Estimated size in bytes of each predicate.
  • Enterprise license information.
  • Max Leased transaction ID.
  • Max Leased UID.
  • CID (Cluster ID).

Here’s an example of JSON for a cluster with three Alpha nodes and three Zero nodes returned from the /state endpoint:

{
  "counter": "22",
  "groups": {
    "1": {
      "members": {
        "1": {
          "id": "1",
          "groupId": 1,
          "addr": "alpha2:7082",
          "leader": true,
          "amDead": false,
          "lastUpdate": "1603350485",
          "clusterInfoOnly": false,
          "forceGroupId": false
        },
        "2": {
          "id": "2",
          "groupId": 1,
          "addr": "alpha1:7080",
          "leader": false,
          "amDead": false,
          "lastUpdate": "0",
          "clusterInfoOnly": false,
          "forceGroupId": false
        },
        "3": {
          "id": "3",
          "groupId": 1,
          "addr": "alpha3:7083",
          "leader": false,
          "amDead": false,
          "lastUpdate": "0",
          "clusterInfoOnly": false,
          "forceGroupId": false
        }
      },
      "tablets": {
        "dgraph.cors": {
          "groupId": 1,
          "predicate": "dgraph.cors",
          "force": false,
          "space": "0",
          "remove": false,
          "readOnly": false,
          "moveTs": "0"
        },
        "dgraph.graphql.schema": {
          "groupId": 1,
          "predicate": "dgraph.graphql.schema",
          "force": false,
          "space": "0",
          "remove": false,
          "readOnly": false,
          "moveTs": "0"
        },
        "dgraph.graphql.schema_created_at": {
          "groupId": 1,
          "predicate": "dgraph.graphql.schema_created_at",
          "force": false,
          "space": "0",
          "remove": false,
          "readOnly": false,
          "moveTs": "0"
        },
        "dgraph.graphql.schema_history": {
          "groupId": 1,
          "predicate": "dgraph.graphql.schema_history",
          "force": false,
          "space": "0",
          "remove": false,
          "readOnly": false,
          "moveTs": "0"
        },
        "dgraph.graphql.xid": {
          "groupId": 1,
          "predicate": "dgraph.graphql.xid",
          "force": false,
          "space": "0",
          "remove": false,
          "readOnly": false,
          "moveTs": "0"
        },
        "dgraph.type": {
          "groupId": 1,
          "predicate": "dgraph.type",
          "force": false,
          "space": "0",
          "remove": false,
          "readOnly": false,
          "moveTs": "0"
        }
      },
      "snapshotTs": "22",
      "checksum": "18099480229465877561"
    }
  },
  "zeros": {
    "1": {
      "id": "1",
      "groupId": 0,
      "addr": "zero1:5080",
      "leader": true,
      "amDead": false,
      "lastUpdate": "0",
      "clusterInfoOnly": false,
      "forceGroupId": false
    },
    "2": {
      "id": "2",
      "groupId": 0,
      "addr": "zero2:5082",
      "leader": false,
      "amDead": false,
      "lastUpdate": "0",
      "clusterInfoOnly": false,
      "forceGroupId": false
    },
    "3": {
      "id": "3",
      "groupId": 0,
      "addr": "zero3:5083",
      "leader": false,
      "amDead": false,
      "lastUpdate": "0",
      "clusterInfoOnly": false,
      "forceGroupId": false
    }
  },
  "maxUID": "10000",
  "maxTxnTs": "10000",
  "maxRaftId": "3",
  "removed": [],
  "cid": "2571d268-b574-41fa-ae5e-a6f8da175d6d",
  "license": {
    "user": "",
    "maxNodes": "18446744073709551615",
    "expiryTs": "1605942487",
    "enabled": true
  }
}

This JSON provides information that includes the following, with node members shown with their node name and HTTP port number:

  • Group 1 members:
    • alpha2:7082, id: 1, leader
    • alpha1:7080, id: 2
    • alpha3:7083, id: 3
  • Group 0 members (Dgraph Zero nodes)
    • zero1:5080, id: 1, leader
    • zero2:5082, id: 2
    • zero3:5083, id: 3
  • maxUID
    • The current maximum lease of UIDs used for blank node UID assignment.
    • This increments in batches of 10,000 IDs. Once the maximum lease is reached, another 10,000 IDs are leased. In the event that the Zero leader is lost, the new leader starts a new lease from maxUID+1. Any UIDs lost between these leases will never be used for blank-node UID assignment.
    • An admin can use the Zero endpoint HTTP GET /assign?what=uids&num=1000 to reserve a range of UIDs (in this case, 1000) to use externally. Zero will never use these UIDs for blank node UID assignment, so the user can use the range to assign UIDs manually to their own data sets.
  • maxTxnTs
    • The current maximum lease of transaction timestamps used to hand out start timestamps and commit timestamps. This increments in batches of 10,000 IDs. After the max lease is reached, another 10,000 IDs are leased. If the Zero leader is lost, then the new leader starts a new lease from maxTxnTs+1 . Any lost transaction IDs between these leases will never be used.
    • An admin can use the Zero endpoint HTTP GET /assign?what=timestamps&num=1000 to increase the current transaction timestamp (in this case, by 1000). This is mainly useful in special-case scenarios; for example, using an existing -p directory to create a fresh cluster to be able to query the latest data in the DB.
  • maxRaftId
    • The number of Zeros available to serve as a leader node. Used by the RAFT consensus algorithm.
  • CID
    • This is a unique UUID representing the cluster-ID for this cluster. It is generated during the initial DB startup and is retained across restarts.
  • Enterprise license
    • Enabled
    • maxNodes: unlimited
    • License expiration, shown in seconds since the Unix epoch.
Note The terms “tablet”, “predicate”, and “edge” are currently synonymous. In future, Dgraph might improve data scalability to shard a predicate into separate tablets that can be assigned to different groups.