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By the point modifications have made their strategy to the legacy database, then you possibly can 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 might 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 important care must be taken if you happen to change the prevailing write behaviour as you might be breaking
an important implicit contract.
Case Research: Incremental area extraction
One in all our groups was working for a shopper whose legacy system had stability points and had turn into tough to take care of and sluggish to replace.
The organisation was seeking to treatment this, and it had been determined that probably the most acceptable manner 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 by one, 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)
- Modifications 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 beneath 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 workforce created a set of companies for a single enterprise area together with the potential for the info
uncovered by these companies to remain in sync with the legacy system.
The companies used Darkish Launching – i.e. not utilized by any customers, as an alternative the companies allowed the workforce to
validate that information migration and synchronisation achieved 100% parity with the legacy datastore.
The place there have been points with reconciliation checks, the workforce might motive about, and repair them guaranteeing
consistency was achieved – with out enterprise impression.
The migration of historic information was achieved by means of a “single shot” information 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 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 information 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 various modifications occurred. Write operations (for the area) have been intercepted and redirected to create/replace/take away
information inside the new area service(s).
This variation made the brand new area service the System of Document for this information, because the legacy information retailer was now not up to date.
Any downstream utilization of that information, reminiscent of reviews, additionally needed to be migrated to turn 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 companies (remodeling them from anemic “information companies”
into true enterprise companies). The entrance finish remained unchanged, and was now utilizing a legacy facade which
redirected implementation to the brand new area service(s).
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