Initialize Inter-Sync Gateway Replications

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    Description — Initializing and running inter-Sync Gateway replication
    Abstract — This content explains how to configure, initialize and run replications between Sync Gateway clusters.
    Related Content — Configuration Details | Admin REST API | Inter-Sync Gateway Replication

    Context Clarification

    This content relates only to inter-Sync Gateway replication in Sync Gateway 2.8+. For documentation on pre-2.8 inter-Sync Gateway replication (also known as SG Replicate) — see Inter-Sync Gateway Replication pre-2.8

    Introduction

    Replications are initialized by submitting a replication definitionglossary icon using either:

    • A 'JSON' configuration file (sync-gateway-config.json)

    • The Admin REST API, using a utility such as curl, or an application such as Postman.

    Wherever they are defined, the elements of a replication definition are the same, with the exception of these Admin REST API only elements:

    • adhoc — Use this to specify that the replication is ad hoc [1].

    • cancel — Use this to cancel on-going replications [1].

    Example 1. Replication Characteristics Highlights
    • Replication highlights

    • Running highlights

    • There are two types of replication: persistent and ad hoc (REST API only).

    • Replications of both types can run in one-shot or continuous replications modes.

    • All replications involve at least one local database.

    • Replications can be configured to purge documents when channel access is revoked (a removal notification is received).

    • Persistent continuous replications can be:

      • Reset — checkpoints can be reset to zero

      • Updated — only the parameter values provided in the PUT request body will be updated

    • Persistent and ad hoc replications can be:

      • Removed — only the replication_id is needed to delete ongoing continuous or one-shot replications.

    • ENTERPRISE EDITION only:

      • Replications can use delta-sync mode, whereby only the changed data-items are replicated.

    • Multiple identical replicators can be initiated on a Sync Gateway node provided each has a unique replication_Id.

    • inter-Sync Gateway replications introduced in Sync Gateway 2.8 as well as SG-Replicate can run on the same node, but you must ensure that they each have a different replication_id.

    • The user under which replication is being run must have read and write access to the data being replicated.

    • Exponential backoff when connection lost; this can be customized using the max_backoff_time configuration setting.

    • replications will continue trying to connect for 30 minutes following authentication failure (including user-invalid/doesn’t exist).

    • Running replications can be stopped. Stopped replications can be (re)Started.

    • If ALL the Sync Gateway nodes in a source or target Sync Gateway cluster go down in the middle of continuous replication, by default, the system should pick up from the last document that was successfully processed by both sides when the replication/cluster is restarted

    • REST ONLY

      • POST databases/{db}/_replication creates a replication using the {rep-id} specified in the body or if none specified, a unique UUID.

      • PUT databases/{db}/_replication/{rep-id} upserts replication {rep-id}.

    • ENTERPRISE EDITION only:

      • Replications are distributed even across all available Sync Gateway nodes and so are not guaranteed to run on their originating node.

      • If a multi-node Sync Gateway cluster loses a subset of sync gateway nodes, the remaining nodes continue replication uninterrupted IF they have been configured to handle the replication (continuous and one-shot replications).

    Replication Definition

    All replications are 'initialized' by a replication definitionglossary icon in the configuration file or Admin REST API and operate within the context of a local database.

    • Configured replications use the database.{db-name}.replications property to add a replication definition to a local database.

    • REST API replications specify the local database and replication identity in the API POST/PUT request. Providing the replication definition parameters in the request body as a JSON string.

    Both scenarios are covered in Example 2. It summarizes the replication definitionglossary icon elements[2], which are covered in more detail in Configuration Details.

    Database-level Settings

    A number of database-level options are also especially relevant to Inter-Sync Gateway Replication, including:

    • sgreplicate_enabled — use this ENTERPRISE EDITION setting to allow the database to participate in Inter-Sync Gateway Replications.

    • delta_sync — use this setting to enable delta-sync replication on the database, it must be set if you want to use delta-sync in your replication definition.

    • sgreplicate_websocket_heartbeat_secs — use this setting to override the default (5 minute) heartbeat interval for websocket ping frames for this database.

    • sync — use this setting to specify the sync function logic — this is an essential part of access-control.

    • unsupported.sgr_tls_skip_verify — use this unsupported option to make development an testing easier by skipping verification of TLS certificates.

    Replication-level Settings

    Example 2. Replication Definition
    • Summary of Parameters

    • Configured Example

    • REST API Example

    This table summarize all the available configurable items. It includes a link to a detailed description of each.

    Name and Link Summary

    REST API ONLY
    Use the Admin REST API’s adhoc parameter to specify that a replication is ad hoc rather than persistent.

    Use batch_size to specify the number of changes to be included in a single batch during replication.

    REST API ONLY
    Use the Admin REST API’s cancel parameter only when you want to want to cancel an existing active replication.

    Use conflict_resolution_type to specify how Sync Gateway should resolve conflicts. By default the automatic conflict resolution policy is applied.

    Use continuous to specify whether this replication will run continuously, or be one-shot.

    Use custom_conflict_resolver to provide the Javascript function used to resolve conflicts if "conflict_resolution_type": "custom".

    Use direction to specify the replication is push, pull or pushAndPull relative to this node.

    Use enable_delta_sync to specify use of delta sync for a replication.

    Use filter to specify the name of the function to be used to filter documents.

    Use max_backoff_time to specify the number of minutes Sync Gateway will spend trying to reconnect lost or unreachable remote targets.

    Use purge_on_removal to specify (per replication) whether removing a channel should trigger a purge.

    Use query_params to specify the key/value pairs to be passed to the filter named in filter.

    Use remote to specify the database endpoint on the remote Sync Gateway custer.

    Use replication_id to specify an identifying name for the replication.

    Use initial_state to specify the state in which to launch the replication.

    This is an example of a replication definition. Its purpose is to illustrate all configurable items in use and so should not be considered a working example.

    It creates a replication with the replication_ID of db1-rep-id1-pull-oneshot on a local database db1-local_, pulling data from a remote database db1-remote.

    "databases": {
     " db1": {                                                (1)
        "bucket":"db1",
        "server": "couchbase://cb-server",
        // ... other DB config ..
        "sgreplicate_enabled": true,                          (2)
        "replications":
          "db1-rep-id1-pull-oneshot":                         (3)
            "direction": "pull",                              (4)
            "remote": "https://example.com:4984/remote_db1",
            "user": "user1",                                  (5)
            "password": "password",
            "batch_size": 1000,                               (6)
            "conflict_resolution_type": "custom",             (7)
            "custom_conflict_resolver": "",                   (8)
            "continuous": false,                              (9)
            "enable_delta_sync": false,                       (10)
            "filter": "sync_gateway/bychannel",               (11)
            "query_params": ["channel.user1"]                 (12)
            "max_backoff_time": 5,                            (13)
            "purge_on_removal": false                         (14)
            "state": "running"                                (15)
        }
      }
    1 All replications are defined at database level within the context of a local database (e.g. DB1)
    2 Opt in to replication
    3 Define the replication_id
    4 Pull changes from the remote database at the specified url.
    5 Authenticate with the provided credentials. This user must have read and write access to both the local and remote databases.
    6 Batch together up to 1000 revisions at a time. This improve replication performance but consumes more memory resources.
    7 Apply a custom conflict resolution policy.
    8 Provide a working Javascript function to apply the required resolution policy.
    9 By setting continuous=false, we are creating a one-shot replication. We could also have omitted this parameter as it defaults to false.
    10 Don’t use delta-sync; the default behavior.
    11 Filter documents by channel.
    12 Replicate only those documents tagged with the channel names "user1".
    13 Wait no more than 5 minutes between retries after network failure; default behavior.
    14 Don’t purge following removal of a channel; the default behavior.
    15 Start the replicator immediately and on Sync Gateway node re(start);. We could also have omitted this parameter as this is the default behavior.

    This is an example of a replication definition as you might submit it to the Admin REST API.using curl. Its purpose is to illustrate all configurable items in use and so should not be considered a working example.

    It creates a replication with the replication_ID of db1-rep-id1-pull-oneshot on a local database db1-local_, pulling data from a remote database db1-remote.

    curl --location --request POST 'http://localhost:4985/db1-local/_replication/db1-rep-id1-pull-oneshot' \ (1)
    --header 'Content-Type: application/json' \
    --dataraw '{
    "replication_id": "db1-rep-id1-pull-oneshot" (2)
            "direction": "pull",                              (3)
            "remote": "https://example.com:4984/remote_db1",
            "user": "user1",                                  (4)
            "password": "password",
            "batch_size": 1000,                               (5)
            "conflict_resolution_type": "custom",             (6)
            "custom_conflict_resolver": "",                   (7)
            "continuous": false,                              (8)
            "enable_delta_sync": false,                       (9)
            "filter": "sync_gateway/bychannel",               (10)
            "query_params": ["channel.user1"]                 (11)
            "max_backoff_time": 5,                            (12)
            "purge_on_removal": false                         (13)
            "state": "running"                                (14)
      "adhoc": false, (15)
      "cancel": false (16)
    }'
    1 All replications take place at database level and in the context of a local database. Here we are setting the replication in the context of db1-local
    2 Define the replication_id
    3 Pull changes from the remote database at the specified url.
    4 Authenticate with the provided credentials. This user must have read and write access to both the local and remote databases.
    5 Batch together up to 1000 revisions at a time. This improve replication performance but consumes more memory resources.
    6 Apply a custom conflict resolution policy.
    7 Provide a working Javascript function to apply the required resolution policy.
    8 By setting continuous=false, we are creating a one-shot replication. We could also have omitted this parameter as it defaults to false.
    9 Don’t use delta-sync; the default behavior.
    10 Filter documents by channel.
    11 Replicate only those documents tagged with the channel names "user1".
    12 Wait no more than 5 minutes between retries after network failure; default behavior.
    13 Don’t purge following removal of a channel; the default behavior.
    14 Start the replicator immediately and on Sync Gateway node re(start);. We could also have omitted this parameter as this is the default behavior.
    15 Setting adhoc=false marks this as a persistent replication. The definition will survive Sync Gateway node restarts. This the default behaviour if this parameter is omitted.+
    16 Set cancel=true to cancel an initialized replication; otherwise you can omit this parameter.

    Generic Constraints

    Replication

    All active nodes in an active cluster must be running Sync Gateway version 2.8+.

    ENTERPRISE EDITION

    All replications are distributed evenly across available nodes. This means they cannot be guaranteed to run on the node from which they originate.

    Access rights

    The user running the replication must have read and write access to the data being replicated. This is not enforced by the system. Use your sync function to ensure a consistent approach is applied across all clusters.

    Mixing Inter-Sync Gateway Replication Versions

    Versions of inter-Sync Gateway replications pre- and post-2.8 can legitimately be in use at the same time, especially during transition. However, you should avoid initializing identical pre-2.8 (SG Replicate) and 2.8+ replications.

    Running Configured Replications

    Replications in the configuration file start automatically whenever Sync Gateway is (re)started. Unless you inhibit this by adding an "initial_state": "stopped" parameter to the replication definition — see: initial_state. You can manually start 'stopped' replication using Starting a replication.

    Example 3. Configured Replications — Continuous and One-shot
    • Continuous

    • One-shot

    //  . . . other configuration entries
    "db1-rep-id1-pull-cont":
      "replication_id": "db1-rep-id1-pull-cont",
      "direction": "pull",
      "continuous": true (1)
      "purge-on-removal": true,
      "remote": "http://user:password@example.com:4985/db1-remote",
      "filter":"sync_gateway/bychannel",
      "query_params": {
        "channels": ["channel1.user1"]
      }
    //  . . . other configuration entries
    1 Make this a continuous replication that remains running, listening for changes to process. Because it is also persistent, it will start automatically following Sync Gateway node restarts (state defaults to running).
    //. . . other configuration entries
    "db1-rep-id3-pull-oneshot":
    "replication_id": "db1-rep-id3-pull-oneshot", (1)
    "direction": "pull",
    "remote": "http://user:password@example.com:4985/db1-remote",
    "filter": "sync_gateway/bychannel",
    "query_params": { "channels": ["channel1.user1"] }
    //  . . . other configuration entries
    1 This a a one-shot replication because the continuous parameter defaults to false.

    Running Admin REST API Replications

    Replications initialized by sending a POST, or PUT, request to the _replication endpoint will start running automatically, unless the "initial_state": "stopped" parameter is specified. with a JSON object defining the replication parameters — as shown in Example 4.

    • You can run multiple replications simultaneously with different replication topologies, provided both databases being synchronized have the same sync function.

    You can submit requests using the curl utility (as in these examples) or an application such as Postman.

    Example 4. Submitting API Requests
    • Continuous Pull Replication

    • One-shot

    • Ad-hoc

    This example initializes a persistent, continuous, replication between a local database and one on a remote Sync Gateway instance.

    curl --location --request POST 'http://localhost:4985/db1-local/_replication/' \
    --header 'Content-Type: application/json' \
    --dataraw '{
      "replication_id": "db1-rep-id1-pull-cont",
      "direction": "pull",
      "continuous": true (1)
      "purge-on-removal": true,
      "remote": "http://user:password@example.com:4985/db1-remote",
      "filter":"sync_gateway/bychannel",
      "query_params": {
        "channels": ["channel1.user1"]
      }
    }'

    This example initializes a persistent, one-shot, replication between a local database and one on a remote Sync Gateway instance.

    The replication will run once, after a short delay to allow the Rest API to start. It will then run once after each Sync Gateway restart and-or when manually initiated using the _replicationStatus endpoint — see Manage Inter-Sync Gateway Replications.

    curl --location --request POST 'http://localhost:4985/db1-local/_replication/' \
    --header 'Content-Type: application/json' \
    --dataraw '{
    "replication_id": "db1-rep-id3-pull-oneshot", (1)
    "direction": "pull",
    "remote": "http://user:password@example.com:4985/db1-remote",
    "filter": "sync_gateway/bychannel",
    "query_params": { "channels": ["channel1.user1"] }
    }'
    1 This a a one-shot replication because the continuous parameter defaults to false.
    curl --location --request POST 'http://localhost:4985/db1-local/_replication/' \
    --header 'Content-Type: application/json' \
    --dataraw '{
      "replication_id": "db1-rep-id1-pull-adhoc",
      "adhoc": true, (1)
      "direction": "pull",
      "purge-on-removal": true,
      "remote": "http://user:password@example.com:4985/db1-remote",
      "filter":"sync_gateway/bychannel",
      "query_params": {
        "channels": ["channel1.user1"]
      }
    }'
    1 Run this replication as an ad hoc one. It will run once only, process all changes but not survive Sync Gateway restarts

    1. This parameter is not available in the configuration file.
    2. The definitions apply to configured and API replications).