Occasion Interception


By the point modifications have made their solution to the legacy database, then you might 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 completely different actions.
For instance a row could possibly be inserted right into a separate Occasions desk to be learn/processed by a brand new part –
while continuing with the write as earlier than (or aborting it).
Word that important care needs to be taken when you change the present write behaviour as you might 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 take care of and gradual to replace.

The organisation was trying to treatment this, and it had been determined that probably the most applicable manner ahead for them was to displace the legacy system with capabilities realised by a Service Based mostly Structure.

The technique that the crew 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 unique utility left.
Different concerns that have 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 modifications to domains being extracted was allowed)
  • Adjustments to the legacy utility have been to be minimised – there was an acute scarcity of retained data 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-Knowledge Layers.

Stage 1 – Darkish launch service(s) for a single area

Firstly the crew created a set of providers for a single enterprise area together with the potential 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 a substitute the providers allowed the crew to
validate that knowledge migration and synchronisation achieved 100% parity with the legacy datastore.
The place there have been points with reconciliation checks, the crew may motive about, and repair them guaranteeing
consistency was achieved – with out enterprise affect.

The migration of historic knowledge was achieved by way of a “single shot” knowledge migration course of. While not strictly Occasion Interception, the continuing
synchronisation was achieved utilizing a Change Knowledge Seize (CDC) course of.

Stage 2 – Intercept all reads and redirect to the brand new service(s)

For stage 2 the crew 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 quite a lot of modifications occurred. Write operations (for the area) have been intercepted and redirected to create/replace/take away
knowledge throughout the new area service(s).

This variation made the brand new area service the System of File for this knowledge, because the legacy knowledge retailer was now not up to date.
Any downstream utilization of that knowledge, resembling experiences, 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).