How Can I use Apache to load balance Marklogic Cluster

Question

Hi I am new to Marklogic and Apache. I have been provided task to use apache as loadbalancer for our Marklogic cluster of 3 machines. Marklogic cluster is currently running on Linux servers.

How can we achieve this? Any information regarding this would be helpful.

Answer 1

You could use mod_proxy_balancer. How you configure it depends what MarkLogic client you would like to use. If you would like to use the Java Client API, please follow the second example here to allow apache to generate stickiness cookies. If you would like to use XCC, please configure it to use the ML-Server-generated or backend-generated "SessionID" cookie.

The difference here is that XCC uses sessions whereas the Java Client API builds on the REST API which is stateless, so there are no sessions. However, even in the Java Client API when you use multi-request transactions, that imposes state for the duration of that transaction so the load balancer needs a way to route requests during that transaction to the correct node in the MarkLogic cluster. The stickiness cookie will be resent by the Java Client API with every request that uses a Transaction so the load balancer can maintain that stickiness for requests related to that transaction.

As always, do some testing of your configuration to make sure you got it right. Properly configuring apache plugins is an advanced skill. Since you are new to apache, your best hope of ensuring you got it right is checking with an HTTP monitoring tool like WireShark to look at the HTTP traffic from your application to MarkLogic Server to make sure things are going to the correct node in the cluster as expected.

Answer 2

Note that even with the client APIs (Java, Node.js) its not always obvious or explicit at the language API layer what might cause a session to be created. Explicitly creating multi statement transactions definately will, but other operations may do so as well. If you are using the same connection for UI (browser) and API (REST or XCC) then the browser app is likely to be doing things that create session state.

The safest, but least flexable configuration is "TCP Session Affinity". If they are supported they will eliminate most concerns related to load balancing. Cookie Session Affinity relies on guarenteeing that the load balencer uses the correct cookie. Not all code is equal. I have had cases where it the load balancer didn't always use the cookie provided. Changing the configuration to "Load Balancer provided Cookie Affinity" fixed that.

None of this is needed if all your communications are stateless at the TCP layer, the HTTP layer and the app layer. The later cannot be inferred by the server. Another conern is if your app or middle tier is co-resident with other apps or the same app connecting to the same load balancer and port. That can be difficult to make sure there are no 'crossed wires' . When ML gets a request it associates its identity with the client IP and port. Even without load balencers, most modern HTTP and TCP client libraries implement socket caching. A great perfomrance win, but a hidden source of subtle random severe errors if the library or app are sharing "cookie jars" (not uncomnon). A TCP and Cookie Jar cache used by different application contexts can end up sending state information from one unrelated app in the same process to another. Mostly this is in middle tier app servers that may simply pass on requests from the first tier without domain knowledge, presuming that relying on the low level TCP libraries to "do the right thing" ... They are doing the right thing -- for the use case the library programmers had in mind -- don't assume that your case is the one the library authors assumed. The symptoms tend to be very rare but catastrophic problems with transaction failures and possibly data corruption and security problems (at an application layer) because the server cannot tell the difference between 2 connections from the same middle tier.

Sometimes a better strategy is to load balance between the first tier and the middle tier, and directly connect from the middle tier to MarkLogic. Especially if caching is done at the load balancer. Its more common for caching to be useful between the middle tier and the client then the middle tier and the server. This is also more analogous to the classic 3 tier architecture used with RDBMS's .. where load balancing is between the client and business logic tiers not between business logic and database.