# Data services DataSource applications can obtain data from other DataSource applications through data services. By using a data service, your DataSource application can request objects based on their subject names without needing to know which other DataSource applications supply the objects. ## Overview A data service definition within Liberator defines which of the Integration Adapters connected to the Liberator will supply the data for a particular subject or set of subjects. You define a data service through [configuration](datasource-datasource-configuration.md), which includes * a name that identifies the service, * a regular expression pattern that defines the objects the service can supply, * the names of the DataSource peers to which requests for the objects are sent. Here are some examples of how data services are configured. **📌 NOTE**\ In practice, you don’t have to define data service configuration in detail for Liberator and Transformer, because the [Caplin Deployment Framework](../deployment-framework/index.md) does a lot of the work for you. These examples are just meant to show you how it all works under the hood. ## Example data service Here’s some Liberator configuration that defines a data service: ``` # Single FX Pricing source add-peer remote-name fxpriceadapter remote-label fxpriceadapter remote-type active end-peer add-data-service service-name fx-prices include-pattern ^/FX add-source-group required add-priority remote-label fxpriceadapter end-priority end-source-group end-data-service ``` First we write an `add-peer ... end peer` configuration item. This defines a DataSource peer connection to an Integration Adapter called fxpriceadapter that supplies FX prices through [active subscriptions](datasource-active-subscription-model.md). We follow this with an `add-data-service ... end-data-service` configuration item. This defines a data service called `fx-prices` that gets its data from the `fxpriceadapter`. The configuration options defined within this item are: * `service-name` This defines the name of the data service (`fx-prices`) * `include-pattern` This regular expression specifies that any subscription request whose subject begins with `/FX` is to be handled by the `fx-prices` data service. * `add-source-group ... end-source-group` defines the DataSource peers that provide the data for the `fx-prices` service. * `required` means that a status stale message or status information message is generated if a DataSource peer within the source group goes down. * `add-priority ... end-priority` defines a priority group (see [Failover using data services](#failover-using-data-services) below). Since in this example there is just one Integration Adapter that supplies the FX prices, the priority group just refers to that one Adapter by its `remote-label fxpriceadapter` in the `add-peer` configuration item. ## Specifying multiple sources of data for a service You can configure a data service to obtain its data from more than one source, like this: ``` add-peer remote-name fxpriceadapterA remote-label fxpriceadapterA remote-type active end-peer add-peer remote-name fxpriceadapterB remote-label fxpriceadapterB remote-type active end-peer add-data-service service-name fx-prices include-pattern ^/FX add-source-group required add-priority remote-label fxpriceadapterA end-priority end-source-group add-source-group required add-priority remote-label fxpriceadapterB end-priority end-source-group end-data-service ``` In this configuration, we’ve defined two DataSource peers, `fxpriceadapterA` and `fxpriceadapterB`, each supplying a different subset of FX instrument prices. The data service configuration is split into two `add-source-group` sections, so that any subscription request whose subject starts with `/FX` is sent to both `fxpriceadapterA` and `fxpriceadapterB` at the same time, and each Pricing Adapter subsequently sends Liberator the updates for its particular subset of FX instruments. **📌 NOTE**\ Because all the DataSource peers are subscribed to at once, this type of data service configuration is best used when the DataSource peer in each source group is responsible for supplying data for a subset of the subject range subscribed to. In the above example, if `fxpriceadapterA` and `fxpriceadapterB` each handled _all_ of the /FX instruments, rather than a subset, Liberator would receive duplicate updates for every FX instrument, which would waste communication bandwidth and processor resources. ## Load balancing using data services You can deploy multiple instances of an Integration Adapter, all supplying the same data, so that no individual Adapter becomes overloaded. The Liberator’s data service is configured to send each successive subscription request to the Adapter that has the smallest number of existing subscriptions: ``` # Load balanced FX Pricing sources add-peer remote-name fxpriceadapter1 remote-label fxpriceadapter1 remote-type active end-peer add-peer remote-name fxpriceadapter2 remote-label fxpriceadapter2 remote-type active end-peer add-data-service service-name fx-prices include-pattern ^/FX add-source-group required add-priority remote-label fxpriceadapter1 remote-label fxpriceadapter2 end-priority end-source-group end-data-service ``` In this configuration, we’ve defined two DataSource peers, `fxpriceadapter1` and `fxpriceadapter2`, each supplying the same set of FX instrument prices. The `add-priority` group within the `fx-prices` data service refers to these two peers. This tells Liberator to route each FX subscription request to whichever of `fxpriceadapter1 and fxpriceadapter2` currently has the least number of existing subscriptions, so it balances the load across these two Pricing Adapters. You can load balance across an arbitrary number of DataSource peers (subject to performance considerations of course). If one of the load-balancing peers subsequently fails (or the connection it it is lost), its subscriptons are shared across the remaining available peers. If more peers are deployed than are required to service the average workload, this configuration also provides an element of redundancy and failover. The default load-balancing algorithm is compatible with all data services except those that route subscriptions for trade and blotter channels. A user’s trade channel and blotter channel subscriptions must be routed to the same Trading Adapter instance in order for the blotter to receive all trading notifications. The default load-balancing algorithm cannot guarantee this. **⚠️ WARNING**\ Don’t use the standard load-balancing algorithm for data services that route updates on trade channels and blotter channels; use load balancing with [source affinity](#source-affinity) instead. ### Source affinity Source affinity is an alternative load-balancing algorithm that distributes [channel](../platform-architecture/channels.md) subscriptions evenly across a set of load-balanced adapters while maintaining the integrity of user sessions. If an integration adapter is designed to work with two or more of a user’s channels, serving data to one channel as a result of activity on another, then load balancing adds an extra layer of complexity. Both of the user’s channels must be allocated to the same adapter instance in the load-balanced set. The standard load-balancing algorithm cannot guarantee this. A trading adapter, for example, serves a user’s trade channel and one or more of the user’s blotter channels. As a result of trading activity conducted over the user’s trade channel, the trading adapter creates a blotter subject for the user and populates it with trading activity data. The blotter subject is available for the user’s blotters to subscribe to, but it’s only available from the trading adapter in which the trading occurred. In a load-balanced environment, where there are several instances of the trading adapter, it’s easy for a user’s blotter channel to be allocated to the wrong trading adapter instance. With source affinity enabled, the load balancer establishes an affinity between a single user and a single instance of an adapter in a load-balanced set. This ensures that all of a user’s channels that are served by a specific load-balanced set will be served by the same adapter instance in that set.
đź“Ś NOTE
The source affinity algorithm has the follow characteristics: * **Deterministic**. The selection of an adapter instance is not random. Given the same starting conditions (username, [affinity key](datasource-data-services-configuration.md#affinity-2) and adapter availability), the algorithm will consistently select the same adapter. * **Persistent**. The selection of an adapter instance for a user’s channel is remembered and reused for all the user’s channels that are configured with the same affinity key. This ensures consistency when other adapters in the load-balanced set go offline or restart. Multiple adapter affinities are supported per user, each distinguished by the affinity key for the source group. Affinities persist under an [affinity key](datasource-data-services-configuration.md#affinity-2) until all live channels configured with that affinity key have been discarded, or until the DataSource application (Liberator or Transformer) is restarted. Affinity data is not shared between DataSource components (Liberator and Transformer).
Source affinity is configured by the [`affinity`](datasource-data-services-configuration.md#affinity-2) configuration option of the [`add-source-group`](datasource-data-services-configuration.md#add-source-group) configuration item. Use of the affinity configuration option of the [`add-data-service`](datasource-data-services-configuration.md#add-data-service) configuration item is deprecated. **See also**: * [Channels](../platform-architecture/channels.md) * [Load-balance adapters using source affinity](datasource-load-balance-adapters-using-source-affinity.md), for detailed configuration instructions and examples * [Data services configuration](datasource-data-services-configuration.md) reference ## Failover using data services You can configure a data service so that it will connect to an alternative DataSource peer if the first choice of peer goes down (becomes unavailable). This technique is called failover. Here’s an example: ``` # Failover of FX Pricing sources add-peer remote-name fxpriceadapter1 remote-label fxpriceadapter1 remote-type active end-peer add-peer remote-name fxpriceadapter2 remote-label fxpriceadapter2 remote-type active end-peer add-data-service service-name fx-prices include-pattern ^/FX add-source-group required add-priority remote-label fxpriceadapter1 end-priority add-priority remote-label fxpriceadapter2 end-priority end-source-group end-data-service ``` Once again we have two Integration Adapters that supply the same FX data: `fxpriceadapter1` and `fxpriceadapter2`. The `fx-prices` data service refers to each Adapter from within a distinct priority group and the order of the priority groups determines the failover priority. So when the Liberator starts up, it connects to `fxpriceadapter1` (provided this Adapter is up and running), and then all subsequent FX subscription requests are sent to `fxpriceadapter1`. If `fxpriceadapter1` becomes unavailable - it could stop working, or the network connection to it could be lost - the Liberator fails over by connecting to `fxpriceadapter2`. Liberator sends requests for all its existing subscriptions, and any subsequent subscription requests, to this alternate Adapter. If you wanted your FX pricing data stream to be extremely resilient, you could have three, or even more, FX Pricing Adapters and configure a separate priority group for each one, so that the Liberator could fail over to each Adapter in turn. It is possible to include more than one adapter in each `add-priority` group. Adapters within each `add-priority` group are load-balanced. If an Adapter fails within an `add-priority` group, then subscriptions will failover to the remaining Adapters in the group. If all Adapters within an `add-priority` group fail, then all subscriptions will failover to the Adapters in the next `add-priority` group. ![LoadBalancing](../../../images/LoadBalancing.png) Failover is a suitable configuration for all data services except those that route subscriptions for trade and blotter channels. Because of relative differences in subscription persistance between trade and blotter subscriptions, an edge case can arise during recovery of a failed Trading Adapter instance when a user’s trade and blotter subscriptions can be routed to different instances of the Trading Adapter. A user’s trade channel subscriptions and blotter channel subscriptions must be routed to the same Trading Adapter instance; see [source affinity](#source-affinity) above. **⚠️ WARNING**\ Don’t use failover for data services that route updates on trade channels or blotter channels; use load balancing with [source affinity](#source-affinity) instead. ## Using data services in your own DataSource applications As we have seen above, Liberator makes extensive use of data services, and so does Transformer. You can also configure data services into your own C-based DataSource applications that have been built using the DataSource C API, and into .NET-based DataSource applications built with the DataSource.NET API. Data services aren’t available in Java-based applications built with the [Caplin Integration Suite](../cis-toolkit/index.md), but this isn’t really a problem because you normally use the Caplin Integration Suite to implement Integration Adapters that _supply_ data to the Caplin Platform (Liberator or Transformer), rather than subscribing to data themselves. --- **See also:** * Reference: [Data services configuration](datasource-data-services-configuration.md) * Reference: [DataSource peers configuration](datasource-datasource-peers-configuration-part-1.md) * How can I... [Load-balance adapters using source affinity](datasource-load-balance-adapters-using-source-affinity.md)