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| Data Migrations Series |
Introduction
This weekend I read the chapter 5 on Data Migration (Planning for
Successful Data Migration) from Brent Dawson’s recently released book "Becoming a Dynamics 365
Finance and Supply Chain Solution Architect" (published by Packt Publishing, available on Amazon). The section on best practices makes many good
points, however some of the practices require further clarifications, while some statements can be questionable as the context associated with them can make an important difference. Overall however the recommendations hold.
Concerining Data Migrations (DM), besides a few teachnical recommendations, the author makes also several architectural recommendations that can be summarized as follows:
(1) put the data into a backup system or database,
if possible, and use that system to the data extraction parts of the DM tasks;
(2) use a Tier 2 system for the majority of the
development of the data packages;
(3) once the data packages validated, they can be
used against production environments;
(4) don’t use the OData protocol for data transfer,
but use the Batch API instead;
(5) don’t use dual-write for DM (technology used for
data integrations), first complete the DM and after that enable the dual-write;
(6) have a backup of the environments involved;
(7) have a good internet connection;
(8) plan an environment for DM (at a 2-tier
environment, distinct from the one used for functional testing);
(9) for the gold configurations have an environment with limited access.
General Aspects
In a Data Migration there are at least 2 systems involved, though in more complex scenarios there can be one more source systems, respectively one or more target systems. At minimum there is a source and a target system.
Ideally, a target production environment should not be used for testing the data migration! On the other side, as long there’s a backup with a given state of the system (e.g. only configuration data, without master or transactional data) a system can be always restored to a previous state. This applies to D365 or to any system for which a database backup and restore can be applied. Even so, as best practice it isn’t recommended to use a production environment for testing as this can increase the complexity of the data migration.
Moreover, the same constraint applies also to the sandbox used for UAT (User Acceptance Testing), given that is supposed to represent at different points in time the same state as the production environment). Thus, at least a third environment will be needed.
There are no hard constraints on the source systems. Ideally, one should use the production source system(s) or environments that resemble the production environments. A read replica of the respective environment(s) will work as well, given that there are typically only reads involved.
The downside of accessing directly a production environment for DM is that the data changes frequently, which makes it more difficult to validate the DM logic – the time factor needing to be considered – data being added, deleted or changed. That’s why an environment with a recent snapshot from production would facilitate the process and would make sure that the DM workloads don’t affect production environment’s performance.
Often, a better alternative would be to have a database in between (aka DM layer) that contains only the data in scope of DM. ETL (Extract Transfer Load) jobs can extract the data on demand and in a consistent manner, this approach assuring a snapshot. This layer can be used to build, test and troubleshoot the DM logic, before Go-Live and after, as issues will be more likely raised by the business and will need to be mitigated.
There are also scenarios in which the direct access to source systems is not possible, a push, respectively a push & pull scenario being needed. If possible, it would be great if the data needed for migration could be exported directly from the source system(s) as needed by the target system(s). In some scenarios this might be achievable, though the bigger the differences in schemas betweeen the systems and the more complex the data, the more transformations are needed, respectively the more difficult it becomes to achieve this. Therefore, moving such logic to an intermediate DM layer would facilitate the DM architecture allowing to address many of the challenges.
Batch API
Using Batch API could be a solution when the source environments allow only API access to the data (thus no direct access over SQL scripting) or when the volume of data makes the alternatives unusable. Indeed, OData seems to be slow or unusable when the volume of data exceeds a given threshold, even if the calls can be partitioned.
Another scenario for Batch API is when the source and target systems need to operate in parallel for a considerable amount of time that would make other approaches unusable. Even if a DM typically involves the replacement of one or more systems, there can be exceptions. Such scenario increases a DM’s complexity by several factors and should be avoided. Even if such scenarios seem to be logical and approachable at first sight, the benefits can be easily outrun by the downsides.
Backup
Hopefully, your organization has a backup and restore strategy for the production and other essential environments! The strategy needs to be extended also for the further environments available during the implementation. It’s also true that until Go-Live the target environments don’t suffer many changes. Ideally, a backup should be taken at least when important changes are made to the systems. This can involve the configuration as well the DM. E.g. a setup would be required after the configuration is completed, when the master data, respectively when the transactional data was migrated. A backup of all the systems involved should be taken before Go-Live.
Gold Configuration
Having a system with the gold configuration (the values used to configure the system) available can indeed facilitate the implementation and there are two main reasons for this. Primarily, the gold configuration allows to build reliable processes around its maintenance and to minimize the risk of having discrepancies between expectations and reality. Secondly, the database with the gold configuration can be used to easily setup a new environment and this might be needed often than thought (e.g. for dry runs).
However, in praxis the technical value is easily overrun by the financial aspect as such an environment is barely used and can involve significant costs. As alternative one can use the DAT legal entity from an available environment for storing the gold configuration common across all the legal entities and easily copy it to the other legal entities. In addition, it’s needed to document the deviations, however it’s recommended to document all configurations and use this as baseline for the post-Go-Live changes.
Indeed, the access to the gold configuration should be restrained as much as possible (e.g. only admin, consultants and/or data owners) and change policies should be enforced. Otherwise, one risks having different configurations between the environments. For Go-Live it is critical that the UAT and Go-Live environments have the same configuration.
Independently of the approach used to maintain the gold configuration, it’s recommended to perform a comparison between UAT and production environments to make sure that there are no differences. The comparison can be handled also via SQL scripts, the effort being well-spent when such comparisons needed to be done several times. Even if the data from production isn’t directly accessible, a snapshot of the production database can be copied in another environment. However, this approach requires a good understanding of the tables and/or entities involved. There will be cases (e.g. module parameters) in which it’s easier to perform a manual comparison.
Wrap Up
Coming back to the recommendations, the only points that require some discussion are (1), (2) and maybe (8), while (9) was discussed above (see 'Gold configuration' section).
The recommendation of putting the data into a backup system or database is too vague. A backup system can mean a backup database that can be accessible typically only over DRBMS or an instance of the system having a copy of the data (which usually implies a RDBMS as well). Besides these, a database can refer to a read replica of source system's database or to a DM layer.
Besides price and performance, the main differences between a Tier-1 and a Tier-2 environment (see also the
Microsoft documentation) rely in the number of VM machines (aka boxes) involved, how the various components are distributed between them, respectively the edition of SQL Server used. Otherwise, for the users the system will look the same. The most important constraint is that a Tier-1 isn't suitable for UAT or performance testing. In other words, the environment will be slow for concurrent use.
If the performance is acceptable, if the volume of data and the number of users is small, a Tier-1 environment can be used for building data packages, performing initial DM dry-runs and other tasks. However, a Tier-2 resembles closer the production environment and if the UAT is performed using such a system, the more likely is to identify and address the bottlenecks related to performance. Unless they accept the costs blindly, the customers will need to trade between performance and costs from the perspective of their requirements and their business context.
