EWD 3 Training Course Part 34: QEWD Resilient Mode

Copyright © 2016 M/Gateway Developments Ltd
EWD 3 Training Course
Part 34
QEWD Resilient Mode
Rob Tweed
Director, M/Gateway Developments Ltd
Twitter: @rtweed

QEWD's Request Queue
• QEWD uses the ewd-qoper8 module to provide
the master process queue and worker dispatch
mechanism
• ewd-qoper8's queue is memory based
– A simple JavaScript array
• If the master process stops unexpectedly
or whilst the system is very busy, you
might lose some request messages

QEWD Audit Information
• By default, QEWD does not keep any history or
audit information, eg:
– A history of incoming request messages
– A history of corresponding responses

QEWD Resilient Mode
• QEWD's built in, optional Resilient Mode
addresses these concerns, at least for
WebSocket/Ajax messages
– Backs up incoming requests to your embedded
database
• Redis or Cache
• Not yet available for GT.M
– Backs up response objects to your embedded
database
– Flags completed messages in the database
• When your message handler's finished() function is invoked

QEWD Recovery
• If the QEWD master process stops and is
restarted
– Any clients/browsers that were previously running an
interactive application (WebSockets or Ajax) will re-
register automatically
• Re-establishes the client's QEWD Session
– On re-registration, QEWD checks to see if that
Session had any uncompleted requests in the
Resilient Mode backup database
• If so, they are re-queued automatically

QEWD Resilient Mode Storage
• The QEWD master process saves any incoming
messages asynchronously to your embedded
Global Storage database, using the Document
Store abstraction
– By default, saved in a document named ewdQueue
– You can opt to change this name
– This is done in parallel to ewd-qoper8's in-memory
queue
• ewdQueue is only used as a backup
• Resilient mode shouldn't significantly affect ewd-qoper8's
performance

QEWD Audit Trail
• The ewdQueue Document can be used as
an audit trail. It contains a record of
– Every incoming request object
– Every outgoing response object
• You can determine the retention period
– Default is 1 hour
– Any records older than the retention period
are automatically purged by QEWD

What about Web/REST Services?
• Web/REST Service HTTP requests are not
currently saved into the ewdQueue Document
• Reason:
– If the master process stops, the connection context
between the client and Express is lost
– So it would not be possible to return a response for a
re-queued HTTP request
• This may change in future
– Particularly where you've linked the client to a QEWD
Session: this would allow re-establishment of client
connection context

Enabling Resilient Mode
• Simple change to your QEWD startup file
• Add the resilientMode property to the
config object
• Setting it to true applies the defaults:
– ewdQueue as the Document Name
– 1 hour retention period
• Can over-ride these via an object instead

Edit your QEWD Startup File
var config = {
managementPassword: 'keepThisSecret!',
serverName: 'My QEWD Server',
port: 8080,
poolSize: 2,
database: {
type: 'cache',
params: {
path: 'c:InterSystemsCache2015-2mgr’
},
resilientMode: true
}
};
var qewd = require('qewd').master;
qewd.start(config);
Default mode
enabled

var config = {
port: 8080,
poolSize: 2,
database: {
type: 'cache',
params: {
},
resilientMode: {
keepPeriod: 3600 * 24 // keep for 24 hours
}
}
};
qewd.start(config);
Specify the required
retention period
in seconds

var config = {
port: 8080,
poolSize: 2,
database: {
type: 'cache',
params: {
},
resilientMode: {
queueDocumentName: 'myAuditTrail'
}
}
};
qewd.start(config);
Save the audit trail
information into a
Document named
myAuditTrail instead

var config = {
port: 8080,
poolSize: 2,
database: {
type: 'cache',
params: {
},
resilientMode: {
queueDocumentName: 'myAuditTrail',
keepPeriod: 3600*24*30 // 30 day retention
}
}
};
qewd.start(config);

Restart QEWD
• Resilient Mode is now enabled
• Logging / backup of queued requests and
responses will now commence

Queue Document Structure
{
'message': {
{{timeStamp}}: {
'token': //EWD Session token,
'content': //queued request message payload
}
},
'pending': {
sessionToken: { {{timeStamp}}: ''}
}
}
Where {{timeStamp}} is a unique index/property created from
the system clock time

Time-stamp format
Logic used is:
var time = process.hrtime();
var ix = time[0] * 1e9 + time[1]; // time in nanoseconds
var timeStamp = new Date().getTime() + '-' + ix;
So the date/time for each audit record can be obtained from:
timeStamp.split('-')[0]

During message processing by worker process,
workerStatus information added:
{
'message': {
{{timeStamp}}: {
'content': //queued request message payload,
workerStatus: 'started' || 'error'
}
},
'pending': {
sessionToken: { {{timeStamp}}: ''}
}
}

When processing by a worker process is
completed, workerStatus will be finished
and pending index record is deleted
{
'message': {
{{timeStamp}}: {
workerStatus: 'finished'
}
}
}

Response objects are logged in the
response array
{
'message': {
{{timeStamp}}: {
workerStatus: 'finished',
response: [
// one or more response objects
]
}
}
}

QEWD Audit Trail
• The ewdQueue Document is a standard
ewd-document-store Document, so it can
be accessed and analysed using the
standard DocumentStore / DocumentNode
APIs.
• You can also inspect it using the qewd-
monitor application
• It can be deleted or manipulated without
affecting QEWD's operation

EWD 3 Training Course Part 34: QEWD Resilient Mode

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EWD 3 Training Course Part 34: QEWD Resilient Mode