Chain on Update


Introduction

The usual deployment for a large replication topology will have the client applications reading from hubs or dedicated consumers in order to spread out the load and off-load search request processing from the masters. Most clients know only about the server they have been configured to communicate with.

Directory Servers are represented by rounded rectangles. The masters are blue, the hubs green, and the externally facing servers are gray. The lines between the servers represent replication agreements and the arrows show the direction of data flow (masters replicate to each other). The thick horizontal lines represent firewalls. “…hubs…” could be many hub replication servers used to spread the load out for many dedicated consumer replicas or other hubs. The externally facing servers could be on the public Internet, or on an extranet, at a remote location or a partner’s site. This works fine for search requests. But if a client has to make a write request, the server will send back a referral to the master. This has a couple of problems:

Another problem with this scenario is password policy - account lockout after unsuccessful attempts. If, for example, your policy says that after 3 tries the user is locked out, that works fine for one server, but then the user can go to another replica and attempt login again, meaning your 3 tries is multiplied by the number of replicas the user can connect to directly.

The replication agreements have been omitted from this diagram for clarity, but they are the same as in the above diagram. This shows a client attempting a write request to a read-only consumer, getting a referral response, and following the referral all the way to the master. In the case of a bind attempt, if the client did follow the referral, the session would be initiated and handled on the master, thus defeating the purpose of the local replica.

One solution to this problem has been the use of an LDAP router or a smart LDAP aware switching device. This router/device sits between the client and its local server. The router/device can redirect write requests to the master and read requests to the local server. Or the router/device may be able to follow the referrals on behalf of the client. There are some problems with this:

Solution - Chain On Update

Each replicated suffix already has 1 local backend. For each one of these, a chaining backend is created. Each chaining backend requires one or more farm servers to chain the operations to, so the immediate suppliers are used. This gives the ability to send updates back through 1 layer of firewall at a time. Of course, all chaining backends could be configured to simply send updates directly to the masters if desired. Chaining also requires a proxy user. In this case, the replication bind DN is used since it already exists on all servers. No additional users need to be created. The suffix is then configured to use the additional chaining backend. The replication plugin itself becomes the entry distribution plugin. Replication provides a function - repl_chain_on_update. This function does the work - read requests are sent to the local backend and write/bind requests are sent to the chaining backend. There are certain cases where distribution is not done. All replicated operations are directed to the local backend, as well as all internal operations. All operations sent by directory manager are directed to the local backend. If the local backend is being initialized, all operations are sent to the chaining backend, not just write/bind operations.

In this diagram, the client sends the write request to the local replica, which then chains the requests up through the hubs back to the master. Chaining uses failover, so if a hub or master is down, another path would be followed. The client sends the write request and gets back the write response as if the server it’s talking to was handling the write itself. In the case of a bind operation, the master would contain the password policy attributes such as number of attempts, lockout state, and unlock time, and this data would be replicated to all other servers in the topology.

Problems with Solution

Latency - there may be a long time lag between sending the write/bind request all the way up to the master and getting it back - there may also be a long time until the client can “read its write”, that is, read back the data it just wrote. However, depending on the speed of replication vs. the speed of chaining, it may be very soon after the client gets the response that the modified data is available on the consumer. While this may not be ideal for all deployments, at least the designer has options.

Directory Manager does not chain - The directory manager user is local to each server, even read-only replicas. Chaining directory manager credentials would essentially give away the directory manager password from one server to another server. Directory Manager updates will affect even read-only replicas. Therefore do not use Directory Manager to make updates to the read-only replicas.

Configuration

Master

The masters do not require very much configuration, only to grant the proxy right to the replication bind DN for the replicated suffix. For example, add this attribute and value to the suffix entry:

aci: (targetattr = "*")(version 3.0; acl "Proxied authorization for database links";    
   allow (proxy) (userdn = "ldap:///cn=Replication Manager,cn=config");)    

Hub or Consumer

Step 1 (Hub and Consumer): the chaining backend must be created on the hub and consumer:

dn: cn=chainbe1,cn=chaining database,cn=plugins,cn=config    
objectclass: top    
objectclass: extensibleObject    
objectclass: nsBackendInstance    
cn: chainbe1    
nsslapd-suffix: <suffix to replicate>
nsfarmserverurl: ldap://supplier1:port supplier2:port ... supplierN:port/ # also, ldaps can be used instead
                                                                          # of ldap for secure connections -
                                                                          # requires the secure port
nsmultiplexorbinddn: cn=Replication Manager,cn=config # or whatever the replica bind DN is on the supplier
nsmultiplexorcredentials: password    
nsCheckLocalACI: on    

NOTE: You must add this entry using an LDAP ADD or MODIFY operation - you must send the clear text password for

nsmultiplexorcredentials: password    

and let the directory server encrypt it. You cannot use a pre-hashed or pre-encrypted password.

Step 2 (Hub Only): the chaining backend plugin must be configured to allow chaining of certain controls:

dn: cn=config,cn=chaining database,cn=plugins,cn=config    
changetype: modify    
add: nsTransmittedControl    
nsTransmittedControl: 2.16.840.1.113730.3.4.18    
nsTransmittedControl: 1.3.6.1.4.1.1466.29539.12    

Step 3 (Hub and Consumer): the suffix must be configured with the new chaining backend and the entry distribution plugin.

dn: cn="<suffix to replicate>",cn=mapping tree,cn=config    
changetype: modify    
replace: nsslapd-state    
nsslapd-state: backend    
-    
add: nsslapd-backend    
nsslapd-backend: chainbe1    
-    
add: nsslapd-distribution-plugin    
nsslapd-distribution-plugin: libreplication-plugin    
-    
add: nsslapd-distribution-funct    
nsslapd-distribution-funct: repl_chain_on_update    

Note: This is for current versions of 389 on linux - for nsslapd-distribution-plugin you can just specify the name of the plugin. On other platforms, you may have to use the absolute path and suffix e.g.

nsslapd-distribution-plugin: /usr/lib64/dirsrv/plugins/libreplication-plugin.so    

Problems During Implementation

Last modified on 7 August 2014