Sync Function API Reference

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      Use Sync Functionss to implement effective data routing and access control in the cloud-to-edge synchronization of enterprise data.
      The Sync Function is crucial to the security of data sync and replication. It is in charge of data validation, access control and routing. This topic provides a reference resource on Sync and its helper functions.

      Other Topics: Sync Function Overview

      access()

      Function

      access(username, channelname)

      Purpose

      Use the access() function to grant a user access to a channel.

      Arguments

      Argument Description

      username

      Must be a string identifying a user, or an array of strings identifying multiple users; the function is applied to each user in the array.

      If the value resolves to null the function result is a no-op.

      channels

      Must be a string identifying a channel name, or an array of strings to specify multiple channel names (for example: (['channel1', 'channel2']); the function is applied to each element in the array.

      If the value resolves to null the function result is a no-op.

      As a convenience, the resolved value of either argument may be null or undefined, in which case nothing happens.

      Context

      You can invoke this function multiple times from within your Sync Function. But note that, invoking it multiple times to grant the same user access to the same channel, will result in negative performance implications.

      Prefix the username argument value with role: to apply this function to a role rather than a user. This grants access to the specified channel(s) for all users assigned that role.

      The effects of all access calls by all active documents are effectively combined in a union, so if any document grants a user access to a channel, that user has access to the channel.

      You can use the all channels wildcard ('*') to grant the user access to all documents in all channels.

      Use

      Example 1. access(username, channel)

      This example shows some valid ways to call access():

      access ("jchris", "mtv"); (1)
      access ("jchris", ["mtv", "mtv2", "vh1"]); (2)
      access (["snej", "jchris", "role:admin"], "vh1"); (3)
      access (["snej", "jchris"], ["mtv", "mtv2", "vh1"]); (4)
      access (null, "hbo");  (5)
      access ("snej", null);
      1 Allow access of single channel to single user
      2 Allow access of multiple channels to single user
      3 Allow access of single channel to multiple users
      4 Allow access of multiple channels to multiple users
      5 The null arguments mean these are treated as no-ops

      channel()

      Function Call

      channel(channelname)

      Purpose

      Use the channel() function to route the document to the named channel(s).

      Arguments

      Argument Description

      channels

      Must be a string identifying a channel name, or an array of strings to specify multiple channel names (for example: (['channel1', 'channel2']); the function is applied to each element in the array.

      If the value resolves to null the function result is a no-op.

      Context

      The channel function can be called zero or more times from the sync function, for any document.

      Channels don’t have to be predefined.
      A channel implicitly comes into existence when a document is routed to it.

      Routing changes have no effect until the document is actually saved in the database, so if the sync function first calls channel() or access(), but then rejects the update, the channel and access changes will not occur.

      As a convenience, it is legal to call channel with a null or undefined argument; it simply does nothing.
      This allows you to do something like channel(doc.channels) without having to first check whether doc.channels exists.

      Use

      Example 2. channel(channelname)

      This example routes all "published" documents to the "public" channel:

      function (doc, oldDoc, meta) {
         if (doc.published) {
            channel("public");
         }
      }

      expiry()

      Function

      expiry(value)

      Purpose

      Use expiry(value) to set the expiry value (TTL) on the document.

      Arguments

      Argument Description

      value

      The value can be specified in two ways:

      • As an ISO-8601 format: date string — or example the 6th of July 2016 at 17:00 in the BST timezone would be 2016-07-06T17:00:00+01:00;

      • As a numeric Couchbase Server expiry value 1

      1 Couchbase Server expiries are specified as Unix time, and if the desired TTL is below 30 days then it can also represent an interval in seconds from the current time (for example, a value of 5 will remove the document 5 seconds after it is written to Couchbase Server).

      Context

      Under the hood, the expiration time is set and managed on the Couchbase Server document (TTL is not supported for databases in walrus mode).

      Impact

      The impact on the resulting document when the expiry value is reached depends on the setting of shared-bucket-access:

      Enabled

      The active revision of the document is tombstoned.

      If there is another non-tombstoned revision for this document (i.e a conflict) it will become the active revision.

      The tombstoned revision will be purged when the server’s metadata purge interval is reached.

      Disabled

      The document will be purged from the database.

      As with the existing explicit purge mechanism, this applies only to the local database; it has nothing to do with replication.

      This expiration time is not propagated when the document is replicated.

      The purge of the document does not cause it to be deleted on any other database.

      Inspect a Document Expiry Value

      You can retrieve a document’s expiration time, as it is returned in the response of GET +/\{db/{doc} use show_exp=true as the querystring.

      curl -X GET "http://localhost:4985/ourdb/ourdoc?show_exp=true" -H "accept: application/json"

      Use

      Example 3. expiry(value)
      expiry("2022-06-23T05:00:00+01:00") (1)
      1 Sets the expiry date to 5am on the 23rd June 2022.

      requireAccess()

      Function

      requireAccess(channels)

      Purpose

      Use the requireAccess() function to reject document updates that are not made by the a user with access to at least one of the given channels, as shown in Example 4

      Arguments

      Argument Description

      channels

      Must be a string identifying a channel name, or an array of strings to specify multiple channel names (for example: (['channel1', 'channel2']); the function is applied to each element in the array.

      If the value resolves to null the function result is a no-op.

      Context

      The function signals rejection by throwing an exception, so the rest of the sync function will not be run.

      Note that requireAccess() will only recognize grants made explicitly using a channel name (not by a wildcard).

      So, if a user was granted access using only the all channels wildcard] (*), then requireAccess('anychannelname')' will fail because the user wasn’t granted access to that channel (only to the * channel).

      Use

      Example 4. requireAccess(channels)
      requireAccess("events"); (1)
      
      if (oldDoc) {
          requireAccess(oldDoc.channels); (2)
      }
      1 Throw an exception unless the user has access to read the "events" channel:
      2 Throw an exception unless the user can read one of the channels in the previous revision’s channels property:

      requireAdmin()

      Function

      requireAdmin()

      Purpose

      Use the requireAdmin() function to reject document updates that are not made by the Sync Gateway Admin REST API.

      Arguments

      There are no arguments.

      Use

      Example 5. requireadmin
      requireAdmin(); (1)
      1 Throw an exception unless the request is sent to the Admin REST API

      requireRole()

      Function

      requireRole(rolename)

      Purpose

      Use the requireRole() function to reject document updates that are not made by user with the specified role or roles, as shown in Example 6.

      Arguments

      Argument Description

      rolename

      Must be a string identifying a role, or an array of strings identifying multiple roles; the function is applied to each role in the array.

      If the value resolves to null the function result is a no-op.

      Note — Role names must always be prefixed with role:; an exception is thrown if a role name doesn’t conform with this rule..

      Context

      The function requires that the user has at least one of the specified roles. If that is not the case it signals rejection by throwing an exception. The rest of the sync function will not be run.

      Use

      Example 6. requireRole(rolename)
      requireRole("admin"); (1)
      
      requireRole(["admin", "old-timer"]); (2)
      1 Throw an error unless the user has the "admin" role:
      2 Throw an error unless the user has one or more of those roles:

      requireUser()

      Function

      requireUser(username)

      Purpose

      Use the requireUser() function to reject document updates that are not made by the specified user or users.

      Arguments

      Argument Description

      username

      Must be a string identifying a user, or an array of strings identifying multiple users; the function is applied to each user in the array.

      If the value resolves to null the function result is a no-op.

      Context

      The function signals rejection by throwing an exception, so the rest of the sync function will not be run.

      When validating a document, you should treat all properties of the doc parameter as untrusted. That is because it is the object that you’re validating. This may sound obvious, but it can be easy to make mistakes, like calling requireUser(doc.owners) instead of requireUser(oldDoc.owners).

      When using one document property to validate another, look up that property in oldDoc, not doc!

      Use

      Example 7. requireUser(username)
      requireUser("snej"); (1)
      
      requireUser(["snej", "jchris", "tleyden"]); (2)
      1 Throw an error if the user is not "snej":
      2 Throw an error if user’s name is not in the list username

      role()

      Function

      role(username, rolename)

      Purpose

      Use the role() function to add a role to a user. This indirectly gives them access to any channels assigned to that role.

      Roles, like users, have to be explicitly created by an administrator.

      Arguments

      Argument Description

      rolename

      Must be a string identifying a role, or an array of strings identifying multiple roles; the function is applied to each role in the array.

      If the value resolves to null the function result is a no-op.

      Note — Role names must always be prefixed with role:; an exception is thrown if a role name doesn’t conform with this rule..

      username

      Must be a string identifying a user, or an array of strings identifying multiple users; the function is applied to each user in the array.

      If the value resolves to null the function result is a no-op.

      Context

      This function affects the user’s ability to revise documents, if the access function requires role membership to validate certain types of changes. Its use is similar to access.

      Nonexistent roles don’t cause an error, but have no effect on the user’s access privileges.

      You can create roles retrospectively. As soon as a role is created, any pre-existing references to it take effect.

      Use

      Example 8. role(username, rolename)
      role ("jchris", "role:admin"); (1)
      role ("jchris", ["role:portlandians", "role:portlandians-owners"]); (2)
      role (["snej", "jchris", "traun"], "role:mobile"); (3)
      role ("ed", null);  (4)
      1 The role admin is assigned to the user
      2 Both the named roles are assigned to the user
      3 The role mobile is assigned to all the named users
      4 No op

      throw()

      Function

      throw()

      Purpose

      Use throw() to prevent a document from persisting or syncing to any other users.

      Arguments

      No arguments

      Context

      You enforce the validity of document structure by checking the necessary constraints and throwing an exception if they’re not met.

      In validating a document, you’ll often need to compare the new revision to the old one, to check for illegal changes in state. For example, some properties may be immutable after the document is created, or may be changeable only by certain users, or may only be allowed to change in certain ways. That’s why the current document contents are given to the sync function, as the oldDoc parameter.

      We recommend that you not create invalid documents in the first place. As much as possible, your app logic and validation function should prevent invalid documents from being created locally. The server-side sync function validation should be seen as a fail-safe and a guard against malicious access.

      Use

      Example 9. throw(forbidden:)

      In this example the sync function disallows all writes to the database it is in.

      function(doc) {
      
         throw({forbidden: "read only!"}) (1)
      
      }
      1 The document update will be rejected with an HTTP 403 "Forbidden" error code, with the value of the forbidden: property being the HTTP status message.
      This is the preferred way to reject an update.