Occasion Interception
By the point adjustments have made their method to the legacy database, then you would argue that it’s too late for
occasion interception.
That mentioned, “Pre-commit” triggers can be utilized to intercept a database write occasion and take totally 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).
Notice that vital care needs to be taken if you happen to change the present write behaviour as it’s possible you’ll be breaking
an important implicit contract.
Case Examine: Incremental area extraction
One in every of our groups was working for a consumer whose legacy system had stability points and had change into tough to take care of and gradual to replace.
The organisation was seeking to treatment this, and it had been determined that essentially the most acceptable means ahead for them was to displace the legacy system with capabilities realised by a Service Based mostly Structure.
The technique that the staff adopted was to make use of the Strangler Fig sample and extract domains, one by one, till there was little to not one of the unique software left.
Different issues 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 adjustments to domains being extracted was allowed)
- Adjustments to the legacy software have been to be minimised – there was an acute scarcity of retained information of the legacy system
Legacy state
The diagram under reveals 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 staff 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 customers, as an alternative the providers allowed the staff to
validate that knowledge migration and synchronisation achieved 100% parity with the legacy datastore.
The place there have been points with reconciliation checks, the staff might cause about, and repair them guaranteeing
consistency was achieved – with out enterprise affect.
The migration of historic knowledge was achieved by a “single shot” knowledge migration course of. While not strictly Occasion Interception, the continued
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 staff 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 numerous adjustments occurred. Write operations (for the area) have been intercepted and redirected to create/replace/take away
knowledge throughout the new area service(s).
This transformation made the brand new area service the System of File for this knowledge, because the legacy knowledge retailer was not up to date.
Any downstream utilization of that knowledge, corresponding to stories, additionally needed to be migrated to change 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).
