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Xenon Frequently-Asked Questions
1. What languages are supported for writing services?
The service runtime is currently written in Java. It can run on JVMs configured with a maximum memory pool (Xmx) of 64MB and still host thousands of service instances -- they are about 360-byte of working set each, with state kept on index caches and disk, transparently.
The production code base is about 13K lines of code.
A basic Go implementation of the service host runtime is implemented, and multiple go Xenon services exist, and a Node.js implementation might be desirable in the future.
2. Do services have descriptions or schema?
A service document is defined by a type. In Java, that would be a
Plain-Old Data Object (PODO) class, with fields, nested PODOs etc. but
without any methods. The runtime enables dynamic discovery through the
/template (for any service instance, just append
get access to the parameters each service is accepting, and their
types). It provides an instance of the service state, with additional
metadata (field types, field documentation, etc).
3. Where do I write business logic?
Business logic is encapsulated in one or more cooperating services. For example, to write a provisioning service for a specific cloud, you re-use the same PODO (data model), write a service, and have it talk to cloud specific provisioning APIs. To other services, or a client, it appears like any other provisioning service.
4. What is the tenancy model?
Service framework instances are processes that can run inside containers, VMs or native OSes. The recommended setup is the following: each tenant has its own process, and an independent group of Xenon nodes, which will give strong isolation in terms of resources on disk, network and cpu. The per-tenant node group can still be part of a provider level node group, monitored by a provider set of service hosts (its recursive!). An alternative is to use a single management group, where Xenon node run service instances across tenants. The RBAC authorization model ensure isolation between tenants by giving access to specific users and resources. In additional the Xenon authorization model gives you runtime scoping on all queries and requests to services using a query specification. If you use fields common to documents to express tenancy, you can sandbox services, per tenant this way. Please see the authorization page for more details
5. Is lucene exposed as a distributed, replicated store?
Lucene is a library hidden behind a core Xenon service, to locally to index and store updates. It is not a distributed store. All replication is done through Xenon mechanisms, entirely independent of lucene. The Lucene semantics are hidden from service implementation and clients. Clients always send updates to the services, never to the store service directly. Of course, queries can be sent to the document store, which will use Lucene underneath to satisfy complex, cross-document relational queries. Both persistence and replication are implemented as separate services, transparently working under the covers to make Xenon service documents durable and replicated across nodes -- but the Lucene document store is not aware of any of this and is used as a local indexed store.
6. Does Xenon expose a key/value store?
Among other things, it does; but we feel it is much more than that: a distributed, fully indexed, multi-versioned and composable collection of service documents, that use policies (control logic) to support common patterns across distributed applications. It is a framework that allows hosting lightweight services across nodes, and selectively provide indexing, durability, and replication for the documents each service represents -- allowing users to pick the mixture of CAP properties they desire. All interaction is done through a service instance instead of a store. This is a key difference with other architectures that expose databases and key-value stores as first class components and expect orchestration to be hosted and run somewhere else.
7. Is replication achieved using PAXOS or a two phase commit protocol?
Service options determine replication and consistency behavior.
Xenon provides strong consensus and "synchronous" replication, using a variant of viewstamped replication (VR) for
services marked with
OWNER_SELECTION. If a service is replicated and
OWNER_SELECTION, all requests are forwarded to an owner node, using
consistent hashing of the service URI
across the node ids. Please start with the multi node tutorial
and the replication protocol page
8. Are strong consistency models supported?
Yes, if a service author enables the OWNER_SELECTION service option. The VR protocol, using consistent hashing to elect a leader, is employed to make sure the majority of the nodes agree on each state update. See the protocol page
9. How do I view the data with out running my application?
The analogy comes from 3 tier applications where a database allows users to read/modify data with out the web server running.Xenon allows us to query the documents via “/core/query-tasks” with out the actual Service running. You can also use the lucene "luke" tool to inspect the index, when xenon is not even started.
10. How should I communicate to remote REST API?
There is no difference in communicating to a service, written in Xenon or otherwise. One can use the same programming constructs like Operation, sendRequest to achieve this. If a service is co-located with another service xenon will use a fast-path but still guarantee strong isolation between sender and receiver.
11. How do I debug my application?
Xenon offers several mechanisms to debug your application in development or production environments. Note that the multi version document store allows you to see per service state evolution. See the Debugging and Troubleshooting page.
12. Is there load balancing / forwarding mechanism? How about DNS?
Xenon has a built in scale out and peer forwarding scheme that allows clients to target any node in a node group, and have the runtime forward the request to the currently assigned "owner" node for the specific service URI. The service must enable the OWNER_SELECTION option. The client can be a regular thin client (curl, a browser, etc) and Xenon will implicitly forward requests as appropriate. There is a DNS Xenon service that can be configured to respond to DNS queries using queries on the Xenon index. Deeper integration of DNS and virtual IP (where a single IP address can be used to represent any Xenon Node) is in the works. Until then, a dedicated node (a edge load balancer for example) should be used to front the Xenon nodes and randomly select nodes based on incoming client requests
13. What will happen to accessing a service when node group is unstable?
If target service is affected by the node group change, any requests to the stateful service will be queued in xenon. Xenon internally retries the request from the node originally received the request from client, and if it cannot get successful response within timeout, the original request from client will timeout.
14. What will happen to the service when nodes are constantly up and down?
In cloud scale environment, it is common that constantly some of the nodes in the group join/leave the node group due to network issue, host shutdown, etc. For example, 1000 nodes are in a node group, and constantly 100 of them are up and down. As long as quorum is met(for example quorum=501), service operates without any problem.
When nodes are constantly up and down, the chance a service is rebalanced to a new owner becomes less likely, as the number of nodes increases. So, with small node groups, rebalancing has a severe impact, whereas it has less of an impact with a lot of nodes. In addition, node group configuration determines how long each node waits for node stability, before requests are allowed to be forwarded. Operators should set the membership stability duration to a small value.
15. How to check OWNER_SELECTED service availability using REST API?
Check target factory service availability on ALL nodes. This can be achieved one request from client using broadcast GET with
For owner selected service, the factory owner returns
available=true and rest of them returns
Therefore, when response contains at least one successful response, the service is available. If all responses are failure, then the service is unavailable for the moment.