Distributed Middleware Reliability & Fault Tolerance Support in System S

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Distributed Middleware Reliability & Fault Tolerance

Support in System S

Rohit Wagle, Henrique Andrade, Kristen Hildrum, Chitra Venkatramani and Michael Spicer

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Group Members

Laksri Wijerathna

Himali Erangika

Erica Jayasundara

Harini Sirisena

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What is this paper is about..? Distribute Middleware reliability and Fault

tolerance support in System S.

Fault-tolerance technique to implementing operations in a large-scale distributed system that ensures that all components will eventually have a consistent view of the system even in the component failure.

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Problems Addressed How to develop a reliable large-scale

distributed system? How to ensure that in a large-scale

distributed system that all the components will have a consistent view of the system even in a component failure?

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Difficulties Multiple components are employed in a

large scale distributed system.

Failure in any single component can have system-wide effects.

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Operations on Large-scale DS Trigger a chain of activities across several

tiers of distributed components.

Example:-Online purchase can trigger

-Web front-end Component-Database system Component-Credit card clearinghouse Component

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In Traditional Transaction-oriented Systems Failure in one or more component require

that all state changes related to the current operation be rolled back across the components.

This approach is cumbersome and may be impossible in cases where components do not have the ability to roll back

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In System S Break distributed operation into a series of

smaller operations (local operations), which is called as single component, which are linked together.

The effect of component failure and restart in the middle of the multi-component operation is limited to that component and its immediate neighbors.

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How to overcome the Problems in System S Never roll back once the first local operation

completed. If local operation fails, only that operation

retried until it completes. Ensure that communication between

components is tolerant to failure and the communication protocol implements a retry policy.

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How to overcome the Problems in System S..

Ensure that each component persists enough data when restarted after a failure, it continues pending requests where the predecessor left off.

If the state of the system changes we adjust the operation as appropriate.

Remote Procedure Calls (RPC) between the component-local operations are stored as work items in a queue, where queue is also saved as part of a local action.

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System S Overview Comprises a middleware runtime system

and application development framework.

System S middleware runtime architecture separates the logical system view from the physical system view.

Runtime contains two components1. Centralized components2. Distributed Management Components

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System S Overview

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System S Components(Centralized Components)Streams Application Manager (SAM)

Centralized gatekeeper for logical system information related to the application running on System S.

System entry point for job management tasks.

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System S Components cont’ (Centralized Components)

Streams Resource Manager (SRM)

Centralized gatekeeper for physical system information related to the software and hardware components that make up a System S instance.

Middleware bootstrapper which does the system initialization, upon administrator request.

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Scheduler (SCH)

Responsible for computing placement decision for applications to be deployed on the runtime systems.

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Name Service (NS)

Centralized component responsible for storing service references which enable inter-component communication.

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Authentication and Authorization Service (AAS)

Centralized component that provide user authentication as well as inter-component cross authentication.

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System S Components cont’ (Distributed Management Components)

Host Controller (HC)

Component running on every application host and is responsible for carrying out all local job management tasks like starting, stopping, monitoring processing element on behalf of the request made by SAM.

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System S Components cont’ (Distributed Management Components)

Processing Element container (PEC)

Hosts the application user code embedded in a processing element.

How to achieve wide reliability in System-S.

Two fundamental building blocks required:

1. Building Block 01

2. Building Block 02:

Architecture

How this is achieved ? Ensuring that,

◦ Remote Procedure Call correctly carried out

◦ Failures convey back to caller

This is almost satisfied existing technologies and some protocols in today,

But , System S uses CORBA as Basic RPC mechanism

Building Block 01

Undying inter component communication infrastructure must be reliable

System S uses IBM DB2 as the data stores.

Building Block 02

The data storage mechanism must be reliable.

Reliability approach of System-S

Convert

Until they succeed

Failures can happen due to◦ Component failure◦ Communication failure

Operations are retried always in the case of failures.

Retries are processed… until1. User cancel the operation2. System shutdown3. logical errors

Failures and Retries

Remote operations always executed. Failures are seen as transient in nature.

(i.e failed component restarted quickly and prime with the state, they held before the failure)

Client ability to transparently retry or back out from pending remote operations.

System-S ensures

1. Devised the Reliability architecture ,to deployable as part of the component design rather than backing into a particular framework as CORBA .

a challenging task Because

◦ Distributed system grow organically ,◦ Different components may choose to represent to

present remote interface with several communication mechanisms.

◦ Component writers can pick different reliability levels for different components

◦ Different infrastructure for components

The Reliability Framework

2. Management of component’s internal state.

Information that required to be maintained by the component for its operation.

Info persisted and restored in the case of failure to recover back

The Reliability Framework…

For every component that maintain an internal state to restore after failure

Following information must be store in the durable data store,1. The components in-core management data

structure2. The serialized asynchronous processing

requests (Work item in the component work queue).

3. The repository of completed remote operations and their associated results

Component State Managemnt

Persisting a component’s in-core data structures need to be engineer in a way that that one

as it should not tied to a particular durable storage solution

The System-s use a paradigm made popular by Hibernate.

Component State Managemnt…

Persisting Asynchronous work item is achieved by◦ Serializing the work items while maintaining their

order of submission.◦ Thus, while retrieving them from data store after

a crash, the work items are scheduled to work in the same sequence

Component State Managemnt…

crash

System-S require to some remote operations to be execute at most once.

That means , same request made multiple times…

Reliable middleware should handle them to ensure that they are harmless or re-issue is flagged and correctly dealt with.

Component Operation Processing

To handle this type of situations, each of external operations is classified as either,

Idempotent:- Multiple invocation do not change remote component’s

internal state But, might be different results.

(Eg: an operation queering the internal state of a component)

Non-Idempotent An operation invocation will yield an internal state

change in the remote component.

Component Operation Processing..

Idempotent in safe retries condition as no change

Concerned much more on Non idempotent operations

For each Non idempotent operation,◦ (Operation Transaction Identifier(OTID) ) field

attached to the argument of the interface)◦ This ensure operation is repeated.

Component Operation Processing..

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SubmitJob Call Processing Block diagram

SAM

CLIENT

Otid COMPLE

TE

OTID SESSION

jOBdESC

submitJob

NO

YES

Retrieve/results

RPCRapo

s

Process request

Save results

TID complete

JOB IDsubmitJob Reliability wrapper

Returnedresults

SAM

Output parameter

Considering Non-Idempotent operation states,

It change the initial state of component But does not

Initiate the request to external components Does not carry out asynchronous processing to complete

the request Non-idempotent code are implemented that are

wrapped within the Database transaction

◦ First Consider this simple non idem potent code handling…

Integrating Database Transaction

1. Begin Network Service(oTid)2. Non-idempotent code3. Log service request result(oTid,results)4. End Network Service

Simple non idempotent code handling…

Non idempotent code handling with database transaction1. Begin Network Service(oTid)2. DB Transction Begin3. Non-idempotent code4. Log service request result(oTid,results)5. DB Transction End(Commit)

6. End Network Service

1. Begin Network Service(oTid)2. DB Transction Begin3. Non-idempotent code4. Log service request result(oTid,results)5. DB Transction End(Commit)6. 4. End Network Service

Case 1

Case 1: if system crashes before 5 State changes are not committed to durable storage Hence maintain consistent state Client requesting the remote operation will continue retrying the

request until complete• Case 2 : if system crashes after 5, but no result send to the client

• Then the framework already committed the log of the service request

• Contains only service otid and the response need to send back to the client

• Reliable protocol layer will just look at the log and reply back with the original result.

Case 2

When middleware performing additional operations when using other components.

Eg: Launching PEs Undergo validation of pre condition Security check

Dispatching PEs can be carried out asynchronously

System S approach is◦ Processing task only after the database

transaction under which under which the task was created to the to the durable repository.

Processing Asynchronous Tasks

Perform synchronously

repository

System S approach is better handle the problems by◦ Execution of a new unit of work on each thread

has to go with reliability approach.◦ But quite complicated to implement.◦ Complexity can be reduced by assumption

Work unit can be scheduled after commit from the original request.

This guarantee work units are executed once.

Processing Asynchronous Tasks…

Interacting with each other is very important.

Framework should handle this interaction. Interactions due to

1. user initiated

2. System initiated

Processing Inter component communication

System S job submission process consists of 6 steps

1. Accepts the job description from the user 2.Check the permission

3. Determine PE placement.

The submitJob Operation

queryAAS

No change in the AAS local state

querySSH

SRM

No state change

Check node availability

4. Update the local state◦ Insert job into SAM’s local tables

5. register the job with AAS (registerJOB operation)

6. deploy PEs But HCs do not in persistent state on restart it does

the state from that.

The submitJob Operation…

change in the AAS local state

change the state of the system

But ,Not a problem

Consider registerJOB operation..(SAM AAS)

What happened if AAS crashes…◦ appears as failed, but two possibilities,

◦ 1. AAS complete the JOB Error, if JOB is already in the system

◦ 2. AAS do not complete the JOB SAM must register the JOB, if JOB already in can retry

Failures

What happened if SAM crashes…◦ may leave the distributed system in a

inconsistent state,In the case of◦ Job may not be existed◦ AAS job might be succeeded

◦ On restart SAM retry to submit operation.◦ (while SAM down ,client trying to submit the

operation).◦ But problem , if re-registering the job again.

Failures……

1. Accepts the job description from the user 2.Check the permission 3. Determine PE placement 4. Update the local state

◦ Insert job into SAM’s local tables 5. Generate oTid, for AAS registerJob queue registration work item

with that id. Commit current state (SAM’s internal tables and work queue) to the

database.

5. register the job with AAS (registerJOB operation)

6. deploy PEs But HCs do not in persistent state on restart it does the state from that.

System-S approach slightly changes,

1. PEREPATATION PHASE

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1. Register the job with AAS using already generated oTid

2. Start a local database Transaction 3. Deploy PEs 4. Commit current state to the database.

System-S approach slightly changes…….

2. REGISTER AND LAUNCH PHASE

With in this approach ,◦ Preparation phase

contains no calls to change the internal state harmless

◦ Register and Launch phase Can repeat many times No problem, if SAM fails

Since Register and Launch retries from the beginning

Since same oTid for same call no danger for registering twice the job

System-S approach slightly changes…….

1. Registering PE For failed PEs

2. Generalizing For correcting the proceeding sections

System S approach is better, handles the fault tolerance in

Tying it all together Retry Policy

Retry ControllerI. Bounded retriesII. Unbounded retries

Fault Detection and Recovery During the normal operation of the System S middleware,

once failures are detected, the recovery process is automatically kick-started.

In System S, failure detection is the responsibility of the SRM component.

Failures are detected in two different ways. Central components are periodically contacted by SRM

to ensure their liveliness. This is done using an application-level ping operation that is built into all the components as a part of our framework.

Moreover, all distributed components communicate their liveliness to SRM via a scalable heartbeat mechanism.

The recovery process is simple and involves only the restart of the failed component or components.

Once a failed component is restarted, its state is rebuilt from information in durable storage before it starts processing any new or pending operations.

First, the component in-core structures

are read from storage.

Next, the list of completed operations is retrieved, followed by re-populating the work queue with any pending asynchronous operations.

Once all the state is populated, the component starts accepting new external requests and the pending requests start being processed.

Any components trying to contact the restarted

component will be able to receive responses and the system will resume normal

operation.

Handling Multiple Failures Able to handle multiple component failures at the

same time without any additional work or coordination.

Failed components can be restarted in any order and will begin processing requests as and when they are restarted.

NB: completion of a pending distributed operation depends on the availability of all components needed to service that operation

The failure of a component after it has completed its part of the distributed operation does not affect the completion of the operation.

EXPERIMENTAL EVALUATION Operation Completion Time

Approach Measure the effect of failures in three

different mocked-up component-graph configurations.

All experiments were conducted with System S running on up to five Linux hosts. Each host contains 2 Intel Xeon 3.4 GHz CPUs with 16GB RAM5IBM DB2

Database as durable storage running on a separate dedicated host.

Two reference applications to test out impose load on the middleware:

Source-Relay-Sink (SRS) Market Data Processing (MDP)

Inspiration

Berkeley’s Recovery Oriented Computing paradigm

Bug free is impossible Lower MTTR (Mean Time To Recover) rather

than increasing MTTF (Mean Time To Failure)

Fault Tolerance in 3 Tier Applications – Vaysburd, 1999.

Related work

Inspiration..

Fault Tolerance in 3 Tier Applications – Vaysburd, 1999.

Client tier should tag requests Server tier should offload state to a

database Database tier alone should be concerned

with reliability.

Related work

1). Replica and consistency management

How to physically setup replicas? How to switch to a different one? How to main consistency?

Disadvantages : Overhead of having replicas Difficulty of ensuring consistency in the

presence of non-idempotent operations.

Related work..

Replica and consistency management ..

1). FT CORBA – OMG, 1998. First standardization effort on fault tolerant

middleware support. Handles distributed non-idempotent request

through service replication and consistency.

Related work..


1). An architecture for Object Replication in Distributed Systems – Beedubail et all 1997.

Hot replicas (multiple copies of a service exist in standby)

Fault tolerance layer, a middleware relays state changes from primary replica to secondary ones to maintain consistency.

Related work..


2). Exactly once end to end semantics in CORBA Invocation across Heterogeneous Fault Tolerance ORBs – Vaysburd & Yajnik, 1999.

Similar to TID approach, however assumption is that in case of failures a replica will pick up the request and a multicast mechanism is used to notify all replicas of state changes.

Related work..


3). DOORS by Bell Labs – 2000. Uses interception to capture inter-

component interactions. FT mainly supported through replication.

Related work..


4). Chubby (Lock Service for loosely coupled distributed systems – 2006) & Zookeeper (Wait free coordination for Internet scale systems -2010).

Useful for group services (where a set of nodes vote to elect a master)

Replicate servers and databases to provide high availability.

Related work..

2). Flexible consistency models

Failure is dealt by relaxing ACID and allowing a temporary inconsistent state.

It has been shown that many applications can actually work under such relaxed assumptions.

Related work..

Flexible consistency models..1). Cluster Based Scalable Network Services –

Fox et all, 1997) BASE (Basically Available, Soft State

Eventual Consistency) model. Doesn’t handle situations where non-

idempotent requests are carried out.

Related work..

Flexible consistency models..2). Neptune – Shen et all, 2003) Middleware for clustering support and

replication management of network services.

Flexible replication consistency support.

Related work..

3). Distributed transaction support

Allow a distributed transaction to roll back in case of failures.

Done at the expense of central coordination and a global roll back mechanism.

Related work..

Gave a mechanism for achieving reliability and fault tolerance in large scale distributed system.

Used in real world middleware – IBM Infosphere Streams.

This approach avoids complex rollbacks and the overhead of maintaining active replicas of components.

Can be implemented as an extension to existing low level distributed computing technologies (CORBA, DCOM)

Conclusion

Support for both stateful and stateless components allowing the system to grow organically while providing different levels of reliability for components (global state consistency).

Low MTTR. Can incorporate other low cost alternatives

for ensuring durability(eg: journaling file systems).

Can tolerate or recover from one or more concurrent failures.

Conclusion..

Future plan is to experiment with alternate durable storage mechanisms and use this mechanism in other distributed middleware.

Conclusion..

Good mechanism for implementing FT in a distributed system, by using middleware.

Unlike traditional FT mechanisms, this approach focuses on converting a distributed operation into component local operations and implementing FT in the communication protocol (reliable RPC).

Test results prove reliable FT. This mechanism is used in IBM’s Infoshpere

Streams enterprise platform, which supports large scale distribution and can handle petabytes of data.

Critique

Distributed Middleware Reliability & Fault Tolerance Support in System S

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