Example: Query Remap Plugin

The sample remap plugin, query_remap.c, maps client requests to a number of servers based on a hash of the request’s URL query parameter. This can be useful for spreading load for a given type of request among backend servers, while still maintaining “stickiness” to a single server for similar requests. For example, a search engine may want to send repeated queries for the same keywords to a server that has likely cached the result from a prior query.

Configuration of query_remap

The query remap plugin will allow the query parameter name to be specified, along with the hostnames of the servers to hash across. Sample remap.config rules using query_remap will look like:

map http://www.example.com/search http://srch1.example.com/search @plugin=query_remap.so @pparam=q @pparam=srch1.example.com @pparam=srch2.example.com @pparam=srch3.example.com
map http://www.example.com/profiles http://prof1.example.com/profiles @plugin=query_remap.so @pparam=user_id @pparam=prof1.example.com @pparam=prof2.example.com

The first @pparam specifies the query param key for which the value will be hashed. The remaining parameters list the hostnames of the servers. A request for http://www.example.com/search?q=apache will match the first rule. The plugin will look for the ``q`` parameter and hash the value ‘apache’ to pick from among srch_[1-3]_.example.com to send the request.

If the request does not include a ``q`` query parameter and the plugin decides not to modify the request, the default toURL ‘http://srch1.example.com/search’ will be used by TS.

The parameters are passed to the plugin’s tsremap_new_instance function. In query_remap, tsremap_new_instance creates a plugin-defined query_remap_info struct to store its configuration parameters. The ihandle, an opaque pointer that can be used to pass per-instance data, is set to this struct pointer and will be passed to the tsremap_remap function when it is triggered for a request.

typedef struct _query_remap_info {
  char *param_name;
  size_t param_len;
  char **hosts;
  int num_hosts;
} query_remap_info;
int tsremap_new_instance(int argc,char *argv[],ihandle *ih,char *errbuf,int errbuf_size)
{
  int i;
  if (argc param_name = strdup(argv[2]);
  qri->param_len = strlen(qri->param_name);
  qri->num_hosts = argc - 3;
  qri->hosts = (char**) TSmalloc(qri->num_hosts*sizeof(char*));
  for (i=0; i num_hosts; ++i) {
    qri->hosts[i] = strdup(argv[i+3]);
  }
  *ih = (ihandle)qri;
  return 0;
}

Another way remap plugins may want handle more complex configuration is to specify a configuration filename as a pparam and parse the specified file during instance initialization.

Performing the Remap

The plugin implements the tsremap_remap function, which is called when TS has read the client HTTP request headers and matched the request to a remap rule configured for the plugin. The TSRemapRequestInfo struct contains input and output members for the remap operation.

tsremap_remap uses the configuration information passed via the ihandle and checks the request_query for the configured query parameter. If the parameter is found, the plugin sets a new_host to modify the request host:

int tsremap_remap(ihandle ih, rhandle rh, TSRemapRequestInfo *rri)
{
  int hostidx = -1;
  query_remap_info *qri = (query_remap_info*)ih;
  if (!qri) {
    TSError("[remap] NULL ihandle");
    return 0;
  }
  if (rri && rri->request_query && rri->request_query_size > 0) {
    char *q, *s, *key;
    //make a copy of the query, as it is read only
    q = (char*) TSmalloc(rri->request_query_size+1);
    strncpy(q, rri->request_query, rri->request_query_size);
    q[rri->request_query_size] = '\0';
    s = q;
    //parse query parameters
    for (key = strsep(&s, "&"); key != NULL; key = strsep(&s, "&")) {
      char *val = strchr(key, '=');
      if (val && (size_t)(val-key) == qri->param_len &&
          !strncmp(key, qri->param_name, qri->param_len)) {
        ++val;
        //the param key matched the configured param_name
        //hash the param value to pick a host
        hostidx = hash_fnv32(val, strlen(val)) % (uint32_t)qri->num_hosts;
        break;
      }
    }
    TSfree(q);
    if (hostidx >= 0) {
      rri->new_host_size = strlen(qri->hosts[hostidx]);
      if (rri->new_host_size new_host, qri->hosts[hostidx], rri->new_host_size);
        return 1; //host has been modified
      }
    }
  }
  //the request was not modified, TS will use the toURL from the remap rule
  return 0;
}