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The next server, please!

What's new in Ocamlnet 3: Highly-available RPC - by Gerd Stolpmann, 2009-11-04

Ocamlnet is being renovated, and there will be soon a first testing version of Ocamlnet 3. The author, Gerd Stolpmann, explains in a series of articles what is new, and why Ocamlnet is the best networking platform ever seen. Experience with Hydro, another RPC implementation for Ocaml using the ICE protocol, has shown that it is advantageous to provide a layer that automatically manages the reaction of the RPC system in case of machine failures. Ocamlnet 3 adds such a layer for SunRPC clients.

The SunRPC client implementation included in past versions of Ocamlnet does not deal in any way with connection failures. The socket error was simply passed through to the caller, and the RPC client had then to be shut down. It was up to the caller how to react on such incidents. For example, one could indicate to the end user that the operation cannot be carried out, or one could retry the call. Programming practice showed that it is cumbersome to develop anew the right reaction for every type of RPC connection used in a larger system. The upcoming Ocamlnet 3 release tries to be better here. On top of the existing Rpc_client module the programmer may now install an Rpc_proxy module that handles socket errors much more nicely.

However, before going into detail here, let us first step back, and look at distributed systems that try to handle machine failures. The assumption here is that a socket error is caused by a malfunctioning (usually dead) machine, and that a simple repetition of the RPC call to the same machine is not the most prospective reaction. Distributed systems that can cope with failures of one or several nodes are called highly available (HA). The development of software that has built-in HA capabilities is known be very difficult, and it is especially useful when there is library support for dealing with machine failures.

Examples of fail-over scenarios

A frequent case is the simple fail-over from one server to an alternate server (see picture). Ideally, the alternate server is a copy of the primary server and operates on exactly the same data set. It is beyond the scope of this article how this can be achieved - possible solutions include hardware-based approaches like switching disks from one machine to the other, or software approaches like replicating all data modifications by means of commit protocols. For Ocamlnet it is only important that the client needs a criterion for when to switch to the other server, and instructions how to switch.

When to switch: Basically, we have the case that the client can autonomously decide whether to contact the secondary server instead of the primary one, and the case that the client needs external help. For example, the client could look at socket errors, and if the number of such errors for a certain server exceeds a maximum, the server is declared to be dead, and the client switches autonomously to the alternate server. This is convenient, but it assumes that the alternate server is at any time a replacement for the primary one. Especially when the fail-over is hardware-based this is not valid - before the alternate server is ready a special fail-over procedure must be run that disconnects the disks from the old server and attaches them to the replacement machine. Also, the old server needs to be isolated (i.e. "zombie machines" are ensured to be unreachable from the rest of the network). There is another reason for relying on external information about when to switch: Often, it is resaonable to monitor the cluster machines by special monitoring daemons. These are in a better position to collect and aggregate information about the liveliness of the cluster machines. For example, such services can contiuously ping the nodes of the cluster and also check which network ports are actually reachable. So it is often advisable to factor out the decision whether a node is alive or dead even if the client could do this on its own.

How to switch: RPC calls that can be simply repeated without changing the meaning are called idempotent. For example, if the client just wants to get the value of a remote variable, the RPC call doing so can be simply repeated as often as necessary to obtain the result. In contrast to that an RPC procedure that modifies a remote variable can often not be repeated (e.g. imagine an integer variable is increased by 1). This case is way more complicated to handle, and one ends often up with a so-called commit protocol. This means that the modification is encapsulated as a transaction that is either completely executed or aborted meaning that the effects are completely nullified. The commit protocol ensures that both the client and the server know whether the transaction is finally done or not done. Also, the commit protcol makes it possible to keep data sets synchronized that are stored on several servers. For Ocamlnet this means that the right reaction is either trivial (redirection of the failed idempotent call to the alternate server), or so complex that generic library support is impossible.

A HA scheme for partitioning data

Often, HA is not the only reason for using a cluster of computers. Another motivation is to increase the total capacity of the system by spreading the load over several systems. In the picture there is an example where the load is shared by four servers. For pure load-balancing there is no need to check for and manage connection failures - before doing an RPC call, the right destination machine is simply selected, and the call is directed to it.

However, load-balancing can be combined with HA. The depicted system allows the client to read-access a dataset that is split into four partitions - the read, blue, gree, and brown partition. Each server has one of these partitions as its primary dataset. For example, when the client needs to read a data item from the blue partition it can go to server 2 because this server has all "blue data" (shown as blue square). In addition to that, all servers also store replicated data from other servers to provide a back-up in the case that a node fails. An easy way to do this would be to store the replica for server k on server k'=(k+1) mod N. However, this leads to the problem that the server k' gets double load when server k fails, foiling the increase of capacity by sharing load. The system in the picture implements a better scheme. Here, the partitions are further subdivided so that equal fractions of any partition are replicated on all machines. For example, the blue partition is split into three smaller parts which are then replicated on the three alternate nodes. When server 2 fails, the client can still find all the "blue data" on the remaining three machines, and because all remaining machines get an equal part of the fail-over load the overall capacity of the system shrinks only by 25%.

This scheme has to be considered for the Ocamlnet library in so far it is no longer always the same node that replaces a failing node. It depends now on the data item which server is the right alternate machine if the primary server fails.

Detecting node outages

When a node goes down, we assume that any pending TCP connection to it hangs, and also that attempts for creating new connections are not responded. After some time, the router usually detects that the node is unavailable, and emits special ICMP packets notifying machines in the network about the outage. However, this takes usually a few minutes. Until then, the cluster system needs other mechanims to detect the unavailablility of the node.

In the literature (including this text) a machine is usually either up or down, and there is nothing in between. In practice, however, the problem of zombie machines really exists, and is not as rare as many think. Often, the cause for hovering between life and death are bad disks - disk accesses still work, but take a multitude of the usual time to complete. Of course, such bad nodes are unusable, and should be counted to the dead nodes, but the problem is that the network ports are still responsive. This does not render the network connectivity tests useless, but one should keep in mind that these tests do not cover all reasons for an outage.

The most basic mechanism for recognizing failed nodes are timeouts. Due to the nature of timeouts it takes some time until such a test can indicate a failure, and this limits the usefulness of timeouts. Nevertheless, timeouts are important even when there are better criteria because they are often the only way to interrupt already established but now hanging TCP connections. We have to distinguish between two kinds of timeouts: Firstly, there are timeouts on individual send and receive operations. In many implementations of network protocols these are the only kind of timeouts. Essentially, these timeouts specify a lower bound on the bandwidth of the connection, and they work independently of the length of the exchanged messages. Secondly, one can also set an upper bound on the total time for the RPC call (including the time for both the request and the response). As RPC is usually run on fast LAN's, and specifications of RPC systems often require upper limits on the latency as seen by the end user (something like "95% of the user requests must be responded within 0.5 seconds"), this second interpretation is the more interesting one. Actually, Ocamlnet only implements this second notion of timeout.

The question is whether there are faster ways of detecting failed nodes. One method is surprisingly simple: Each node running an RPC client pings the nodes with the RPC servers the clients are connected to. A repeatedly failed ping is considered as a node outage. Ocamlnet 3 does not (yet) implement such a component, but the new Rpc_proxy layer is prepared to take external information about outages into account. (For an implementation look at Hydromon. Although written for ICE instead of SunRPC it is straight-forward to port Hydromon to Ocamlnet's SunRPC implementation.) Even on busy servers this method is able to detect bad nodes within a few seconds.

Another idea is to share information about good and bad nodes as much as possible. If one connection runs into a timeout and the node is considered as failed, this information should be made available to all threads, and if possible even to all processes of the node. This avoids further attempts to connect to the failed node, and one can even cancel pending calls to this node.

Proxies: Configurable connection management

After this lengthy foreword, let's look at the actual implementation of connection management in Rpc_proxy. The name, "proxy", reminds us of the ideal of RPC: A remote call should be as simple and safe to conduct as a local procedure call, and the "proxy" is the local agent dealing with most of the complexities of remote calls, so that they look as much as possible like local calls.

This module is divided into three parts:

  • Rpc_proxy.ReliabilityCache remembers which nodes and which network ports failed in the past, and decides on this information which nodes are considered to be down.
  • Rpc_proxy.ManagedClient is an encapsulation of the simpler Rpc_client that is able to reconnect to a service. Effectively, a ManagedClient is a single TCP connection to a remote service that can be reestablished after an error.
  • Rpc_proxy.ManagedSet is a set of ManagedClients for the same RPC service that controls how many RPC calls are routed over each managed client, and that also implements the logic for skipping bad clients.

The generator for the language mapping, ocamlrpcgen, has also been extended. It emits now a functorized version of the client wrappers. For example, for the RPC program P the generator would output a functor

module Make'P : Rpc_client.USE_CLIENT -> sig ... end
where the returned module includes for every procedure p a call wrapper
val p : C.t -> argument -> result
Here, C is the input module Rpc_client.USE_CLIENT. This means one can inject any client implementation that at least implements Rpc_client.USE_CLIENT, and is not restricted to the default implementation Rpc_client. Of course, the enhanced client implementation Rpc_proxy also provides the Rpc_client.USE_CLIENT functionality, so it can be substituted here:
module P = Make'P(Rpc_proxy.ManagedClient)
The user is free, however, to pass any module as input here that meets the formal requirements - for example, one could pass further improved or derived versions of Rpc_proxy.ManagedClient. (Of course, the output generated by the new ocamlrpcgen is compatible with what earlier versions emitted. For users it is not required to switch to the functorized version.)

Comparing basic clients and managed clients

Basic clients as provided by Rpc_client live as long as the underlying TCP connection exists. This means the connection is created at the beginning of the client's lifetime, and the client becomes unsuable when the connection ends (either programmatically, or because of socket errors). In contrast to this, a managed client can establish the connection again when needed - but only to the same host and port. (Fail-overs to other hosts/ports are implemented in ManagedSet, see below.)

The basic clients are configured after being created - by invoking configuration functions like configure, set_exception_handler or set_auth_methods. This has the advantage that the configuration can be changed during the lifetime of the client. For managed clients, however, we require that the configuration must be fully given at creation time. The point is here that the managed client internally creates a series of basic clients, and immediately needs to know how these clients are to be configured.

For example, this piece of code creates a managed client:

let mconfig = 
    ~programs:[ P._program ]
    ~msg_timeout: 10.0
let mclient =
    (Rpc_client.Internet(Unix.inet_addr_of_string "", 9005))
    (esys : Unixqueue.event_system)

Note that the TCP connection is not immediately created, but only when the first remote procedure is called. This is on line with the automatic reconnection feature: When the connection is closed or crashes, the next procedure call triggers that the connection is established again.

In this example, we have only configured a message timeout - the client will signal a timeout when the response takes more than 10 seconds to arrive. Normally, the timeout does not mean that the connection is considered as broken. We have to explicitly configure this feature - the proxy layer cannot know by itself what possible reasons for timeouts are. This is just another argument of create_mclient_config:

   ... ~msg_timeout_is_fatal:true ... ()

A fatal error is recorded, and may, depending on the configuration of the ReliabilityCache, disable the server host and/or server port for some time. The ManagedSet layer will recognize the error situation, and take this information into account when selecting good server endpoints for future RPC calls - but more on that below. On the level of ManagedClient it is only important to define which malfunctions are considered as fatal errors that may trigger the fail-over to other server nodes. As a ManagedClient is always only connected with one server node it is pointless to decide what to do in case of node failures.

There are two more connection management features:

  • One can set an idle timeout. Unused TCP connections are closed after the idle timeout period is over. The idea is to save server resources by closing unused connections - which often also means that the thread or process in the server can be released. Also, some network configurations do not tolerate that connections remain unused for longer periods of time.
  • One can demand to ping the service before sending the first RPC request. The background is that servers usually accept TCP connections before knowing whether there are really enough resources to handle them (i.e. free threads or processes). So it may happen that a TCP connection can be established, but it turns out to be immediately dead. By requesting one ping before really using the connection one can detect this problem.

Once the ManagedClient is configured and created, it can be used in a similar way as the older Rpc_client. For example, to call do_something we would write

let result = P.do_something mclient argument
(given that P is the applied functor from above). The asynchronous version, P.do_something'async is also available.

The ReliabilityCache

The reliability cache is the instance that decides whether a server host or server port is available or not. The cache implements a logic for disabling servers when RPC clients recently reported fatal errors. Higher RPC levels such as ManagedSet can then interpret this information and fail-over to alternate servers. In the bigger picture outlined in the first half of this article the cache plays primarily the role of the autonomous fail-over trigger. In addition to that, there is also a hook for plugging in external sources for recognizing dead nodes.

The reliability cache is notified by the managed clients whether RPC calls lead to fatal errors or not. The cache disables the host or port of the server when a certain number of fatal errors occur in sequence. As we don't have a better idea for how long to disable, the cache simply implements a time-based logic where the length of the time period the server is assumed to be down depends on the number of fatal errors: Following fatal errors double the duration of the assumed unavailability until a maximum duration is reached. (A justification for this logic are server overloads: When a server cannot be contacted because of too much load, it is reasonable to delay the addition of load, and to consider the delay as a function of past slowness.)

There is usually only one reliability cache per process (the "global" cache). This is reasonable because the information about failures should be spread as far as possible. Nevertheless, each ManagedClient can configure its own criteria when to disable servers. The function

val derive_rcache : rcache -> rcache_config -> rcache
makes this possible: A new cache is created with a new configuration, but the cache shares error data with a parent cache. For example, this could be used to set the hook for getting external liveliness information:
let rconfig =
    ~availability:(fun rcache sockaddr -> ...)
let rc =

Right now, there is no mechanism to make the mentioned error counters accessible across process boundaries. This may be added in future versions of Ocamlnet.

The ManagedSet of RPC clients

Finally, we come to the layer that represents possible connections to several server nodes, and that decides what to do when one of the servers dies.

A managed set of clients is configured by specifying an array of remote addresses (usually corresponding to the available server nodes). For each of the addresses the set may create a managed client. Basically, there are two scenarios: Either the addresses are seen as alternate server endpoints (when the first one fails try the second etc.), or they are seen as equivalent, and it is tried to distribute the load over all server endpoints that are alive. This translates into the two policies managed sets can operate under: `Failover and `Balance_load.

For example, this managed set specifies a fail-over policy: When fails, it tries to contact instead.

let sconfig = 
let mset =
    [| Rpc_client.Inet(Unix.inet_addr_of_string "", 9009), 100;
       Rpc_client.Inet(Unix.inet_addr_of_string "", 9009), 100;
    (esys : Unixqueue.event_system)

The number 100 is the maximum number of simultaneous connections to this endpoint. The managed set automatically increases the number of connections to the selected endpoint when the load grows (where the load is the number of simultaneously submitted RPC calls - remember this is an asynchronous RPC implementation, and it is able to handle several RPC calls at the same time). When the maximum is reached, further connections attempts are not rejected, but the alternate endpoint is then used instead. (So `Failover does not prevent that the alternate endpoint is used, it only specifies the preference to use the first endpoint as long as possible.)

If we had passed ~policy:`Balance_load instead, the managed set would create connections to both servers, and try to achieve that the number RPC calls directed to each of the servers is roughly the same.

The user code calls mset_pick to get the next managed client according to these selection rules:

let mclient, index = 
  Rpc_proxy.ManagedSet.mset_pick mset
There is also the optional from argument allowing to restrict the endpoints from which the client must be chosen. E.g.
let mclient, index = 
  Rpc_proxy.ManagedSet.mset_pick ~from:[2;3] mset
would take either the endpoint at index 2 or the endpoint at index 3 from the endpoint array, and return the actually selected index. This is useful for fail-over scenarios where the node is a function of the input data - like in the presented partitioning example.

Managed sets do not repeat failed RPC calls by themselves. They only record failures in the rcache, and if enough failures happened, the problematic server is no longer considered as alive when submitting new calls. For idempotent calls, however, there is some support for automatic repetition. But first let's have a closer look at how the clever partitioning scheme from the above picture could be implemented.

Sketch how to implement partitioned data sets

In this example, we had four servers:
let server1 = 
  Rpc_client.Inet(Unix.inet_addr_of_string "", 7654)
let server2 = 
  Rpc_client.Inet(Unix.inet_addr_of_string "", 7654)
let server3 = 
  Rpc_client.Inet(Unix.inet_addr_of_string "", 7654)
let server4 = 
  Rpc_client.Inet(Unix.inet_addr_of_string "", 7654)
In order to access the data item with key k : key we define two hash functions, one for the primary partitioning scheme, and one for the secondary scheme:
val h_primary   : key -> int      (* actually: key -> {0..3} *)
val h_secondary : key -> int      (* actually: key -> {0..3} *)
For example, one could define these as:
let h_primary (k : string) =
  (Char.code (Digest.string k).[0]) land 3

let h_secondary (k : string) =
  let h1 = h_primary k in
  let rec attempt j =
    let k' = k ^ string_of_int j in
    let h2 = (Char.code (Digest.string k').[0]) land 3 in
    if h2 = h1 then (
      if j = 4 then
        (h1+1) mod 4     (* ensure that the loop terminates *)
        attempt (j+1)
    ) else 
  attempt 0
Now, we create our managed set:
let sconfig = 
let mset =
    [| server1, 100;
       server2, 100;
       server3, 100;
       server4, 100;
    (esys : Unixqueue.event_system)
When trying to get the record for key k, we first compute the two nodes that store this record:
let n1 = h_primary k
let n2 = h_secondary k
Finally, we pick a managed client from the managed set. We have to restrict the set of available servers to those where the paritioning functions say they store the record. So we get
let mclient, index =
  Rpc_proxy.ManagedSet.mset_pick ~from:[n1;n2] mset

The mclient can then be used for calling a remote procedure. Remember that managed clients/sets do not automatically repeat failed calls, so we still can get socket errors etc. The error is only recorded, and when the server looks too faulty, it is finally disabled, and the managed set will no longer pick it. Such a call could look like:

let result = P.get_file mclient file_id

Idempotent calls can be repeated without changing the semantics. For this reason, managed sets have a little helper to do so:

let result =

This would repeat the get_file call in case of errors (up to a configurable maximum). Note that we have to pass the asynchronous version of get_file as argument although the resulting call is synchronous.

Summarized, the proxy module achieves that the actual fail-over logic is implemented within ManagedSet and the layers below it. The user code can rather focus, as shown, on describing which server to chose, and does not need to think in actions ("if this fails then try that").

Where to get Ocamlnet 3

There is no official release yet, not even an alpha release for developers. In order to get it, one has to check out the Subversion repository (use the svn command on this URL, or click on it to view it with your web browser - most of the discussed code lives in src/rpc).

Gerd Stolpmann works as O'Caml consultant. He is accepting new customers!
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