By the point adjustments have made their method to the legacy database, then you could possibly argue that it’s too late for
occasion interception.
That stated, “Pre-commit” triggers can be utilized to intercept a database write occasion and take totally different actions.
For instance a row may very well be inserted right into a separate Occasions desk to be learn/processed by a brand new element –
while continuing with the write as earlier than (or aborting it).
Word that vital care must be taken if you happen to change the present write behaviour as you could be breaking
an important implicit contract.
Case Examine: Incremental area extraction
One in every of our groups was working for a shopper whose legacy system had stability points and had develop into tough to keep up and sluggish to replace.
The organisation was trying to treatment this, and it had been determined that essentially the most acceptable method ahead for them was to displace the legacy system with capabilities realised by a Service Primarily based Structure.
The technique that the workforce adopted was to make use of the Strangler Fig sample and extract domains, one after the other, till there was little to not one of the authentic software left.
Different concerns that had been in play included:
- The necessity to proceed to make use of the legacy system with out interruption
- The necessity to proceed to permit upkeep and enhancement to the legacy system (although minimising adjustments to domains being extracted was allowed)
- Modifications to the legacy software had been to be minimised – there was an acute scarcity of retained information of the legacy system
Legacy state
The diagram under exhibits the structure of the legacy
structure. The monolithic system’s
structure was primarily Presentation-Area-Information Layers.
Stage 1 – Darkish launch service(s) for a single area
Firstly the workforce created a set of providers for a single enterprise area together with the aptitude for the info
uncovered by these providers to remain in sync with the legacy system.
The providers used Darkish Launching – i.e. not utilized by any shoppers, as an alternative the providers allowed the workforce to
validate that knowledge migration and synchronisation achieved 100% parity with the legacy datastore.
The place there have been points with reconciliation checks, the workforce may purpose about, and repair them guaranteeing
consistency was achieved – with out enterprise influence.
The migration of historic knowledge was achieved by a “single shot” knowledge migration course of. While not strictly Occasion Interception, the continuing
synchronisation was achieved utilizing a Change Information Seize (CDC) course of.
Stage 2 – Intercept all reads and redirect to the brand new service(s)
For stage 2 the workforce up to date the legacy Persistence Layer to intercept and redirect all of the learn operations (for this area) to
retrieve the info from the brand new area service(s). Write operations nonetheless utilised the legacy knowledge retailer. That is
and instance of Department by Abstraction – the interface of the Persistence Layer stays unchanged and a brand new underlying implementation
put in place.
Stage 3 – Intercept all writes and redirect to the brand new service(s)
At stage 3 a lot of adjustments occurred. Write operations (for the area) had been intercepted and redirected to create/replace/take away
knowledge inside the new area service(s).
This alteration made the brand new area service the System of Report for this knowledge, because the legacy knowledge retailer was not up to date.
Any downstream utilization of that knowledge, resembling stories, additionally needed to be migrated to develop into a part of or use the brand new
area service.
Stage 4 – Migrate area enterprise guidelines / logic to the brand new service(s)
At stage 4 enterprise logic was migrated into the brand new area providers (remodeling them from anemic “knowledge providers”
into true enterprise providers). The entrance finish remained unchanged, and was now utilizing a legacy facade which
redirected implementation to the brand new area service(s).
