---
uid: Lucene.Net.Replicator
summary: *content
---
The Replicator allows replicating files between a server and client(s). Producers publish revisions and consumers update to the latest revision available. ReplicationClient is a helper utility for performing the update operation. It can be invoked either manually or periodically by starting an update thread. HttpReplicator can be used to replicate revisions by consumers that reside on a different node than the producer.
The replication framework supports replicating any type of files, with built-in support for a single search index as well as an index and taxonomy pair. For a single index, the application should publish an IndexRevision and set IndexReplicationHandler on the client. For an index and taxonomy pair, the application should publish an IndexAndTaxonomyRevision and set IndexAndTaxonomyReplicationHandler on the client.
When the replication client detects that there is a newer revision available, it copies the files of the revision and then invokes the handler to complete the operation (e.g. copy the files to the index directory, sync them, reopen an index reader etc.). By default, only files that do not exist in the handler's current revision files are copied, however this can be overridden by extending the client.
Because there are a number of different hosting frameworks to choose from on .NET and they don't implement common abstractions for requests and responses, the ReplicatorService provides abstractions so that it can be integrated easily into any of these frameworks.
To integrate the replicator into an existing hosting framework, the xref:Lucene.Net.Replicator.Http.Abstractions.IReplicationRequest and xref:Lucene.Net.Replicator.Http.Abstractions.IReplicationResponse interfaces must be implemented for the chosen framework.
Below is an example of how these wrappers can be implemented for the ASP.NET Core framework. The example only covers the absolute minimum needed in order for it to become functional within ASP.NET Core.
It does not go as far as to implement custom middleware, action results for controllers or anything else, while this would be a natural to do, such implementations extends beyond the scope of this document.
The first thing to do is to wrap the ASP.NET Core Request object in a class that implements the xref:Lucene.Net.Replicator.Http.Abstractions.IReplicationRequest interface. This is very straight forward.
// Wrapper class for the Microsoft.AspNetCore.Http.HttpRequest
public class AspNetCoreReplicationRequest : IReplicationRequest
{
private readonly HttpRequest request;
// Inject the actual request object in the constructor.
public AspNetCoreReplicationRequest(HttpRequest request)
=> this.request = request;
// Provide the full path relative to the host.
// In the common case in AspNetCore this should just return the full path, so PathBase + Path are concatenated and returned.
//
// The path expected by the ReplicatorService is {context}/{shard}/{action} where:
// - action may be Obtain, Release or Update
// - context is the same context that is provided to the ReplicatorService constructor and defaults to '/replicate'
public string Path
=> request.PathBase + request.Path;
// Return values for parameters used by the ReplicatorService
// The ReplicatorService will call this with:
// - version: The index revision
// - sessionid: The ID of the session
// - source: The source index for the files
// - filename: The file name
//
// In this implementation a exception is thrown in the case that parameters are provided multiple times.
public string QueryParam(string name)
=> request.Query[name].SingleOrDefault();
}Secondly the ASP.NET Core Response object is wrapped in a class that implements the xref:Lucene.Net.Replicator.Http.Abstractions.IReplicationResponse interface. This is also very straight forward.
// Wrapper class for the Microsoft.AspNetCore.Http.HttpResponse
public class AspNetCoreReplicationResponse : IReplicationResponse
{
private readonly HttpResponse response;
// Inject the actual response object in the constructor.
public AspNetCoreReplicationResponse(HttpResponse response)
=> this.response = response;
// Getter and Setter for the http Status code, in case of failure the ReplicatorService will set this
// Property.
public int StatusCode
{
get => response.StatusCode;
set => response.StatusCode = value;
}
// Return a stream where the ReplicatorService can write to for the response.
// Depending on the action either the file or the sesssion token will be written to this stream.
public Stream Body => response.Body;
// Called when the ReplicatorService is done writing data to the response.
// Here it is mapped to the flush method on the "body" stream on the response.
public void Flush() => response.Body.Flush();
}This part is not nessesary, however by providing a extension method as a overload to the ReplicatorService Perform method that instead takes the ASP.NET Core HttpRequest and HttpResponse response objects, it's easier to call the ReplicatorService from either ASP.NET Core MVC controllers, inside of middleare or for the absolute minimal solution directly in the delegate parameter of a IApplicationBuilder.Run() method.
public static class AspNetCoreReplicationServiceExtentions
{
// Optionally, provide a extension method for calling the perform method directly using the specific request
// and response objects from AspNetCore
public static void Perform(this ReplicationService self, HttpRequest request, HttpResponse response)
=> self.Perform(
new AspNetCoreReplicationRequest(request),
new AspNetCoreReplicationResponse(response));
}Now the implementation can be used within ASP.NET Core in order to service Lucene.NET Replicator requests over HTTP.
In order to enable replication of indexes, the xref:Lucene.Net.Index.IndexWriter that writes the index should be created with a xref:Lucene.Net.Index.SnapshotDeletionPolicy.
IndexWriterConfig config = new IndexWriterConfig(...ver..., new StandardAnalyzer(...ver...));
config.IndexDeletionPolicy = new SnapshotDeletionPolicy(config.IndexDeletionPolicy);
IndexWriter writer = new IndexWriter(FSDirectory.Open("..."), config);For the absolute minimal solution we can wire the xref:Lucene.Net.Replicator.Http.ReplicationService up on the server side as:
LocalReplicator replicator = new LocalReplicator();
ReplicatorService service = new ReplicationService(new Dictionary<string, IReplicator>{
["shard_name"] = replicator
}, "/api/replicate");
app.Map("/api/replicate", builder => {
builder.Run(async context => {
await Task.Yield();
service.Perform(context.Request, context.Response);
});
});Now in order to publish a Revision call the Publish() method in the xref:Lucene.Net.Replicator.LocalReplicator:
IndexWriter writer = ...;
LocalReplicator replicator = ...;
replicator.Publish(new IndexRevision(writer));On the client side create a new xref:Lucene.Net.Replicator.Http.HttpReplicator and start replicating, e.g.:
IReplicator replicator = new HttpReplicator("http://{host}:{port}/api/replicate/shard_name");
ReplicationClient client = new ReplicationClient(
replicator,
new IndexReplicationHandler(
FSDirectory.Open(...directory...),
() => ...onUpdate...),
new PerSessionDirectoryFactory(...temp-working-directory...));
//Now either start the Update Thread or do manual pulls periodically.
client.UpdateNow(); //Manual Pull
client.StartUpdateThread(1000, "Replicator Thread"); //Pull automatically every second if there is any changes.From here it would be natural to use a xref:Lucene.Net.Search.SearcherManager over the directory in order to get Searchers updated automatically. But this cannot be created before the first actual replication as the xref:Lucene.Net.Search.SearcherManager will fail because there is no index.
We can use the onUpdate handler to perform the first initialization in this case.