Archives For High Availability

Ge0-HA-Map

Geo-HA Service Fabric Cluster

One of the biggest challenges that we face when we build an Internet-scale solution, is high availability across geographic locations (Geo-HA). Why is this important? Well, there can be a few different reasons. The most common reason, is to be able to survive data center outages. Another reason, is to bring services closer to end users so that we can provide good user experiences.

Geo-HA brings challenges to the table. For example, should we use an Active-Passive or Active-Active strategy for data across regions? Keeping in mind that Active-Active is difficult to get right, we need to take time to analyze and to make the correct choices. We need to consider our Disaster recovery (DR) plan, target RPO and RTO. Azure has a whole bunch of mechanisms for replication, backup and monitoring, so how do we decide what’s the right combination?

Today’s Internet-scale services are built using microservices. Service Fabric is a next-generation middleware platform used for building enterprise-class, Tier-1 services. This microservices platform allows us to build scalable, highly available, reliable, and easy to manage solutions. It addresses the significant challenges in developing and managing stateful services. The Reliable Actors API is one of two high-level frameworks provided by Service Fabric, and it is based on the Actor pattern. This API gives us an asynchronous, single-threaded programming model that simplifies our code while still providing the advantages of scalability and reliability guarantees offered by Service Fabric.

A Service Fabric cluster is HA within its geographic region by default. Thinking about our heritage of on premise data centers, we’ve poured thousands of man-hours to deploy Disaster Recovery sites in secondary physical locations, because we know that everything is possible. Over the past few years, we’ve experienced many interesting scenarios, for example, a cut cable, or a faulty DNS entry broke the Internet. So why should we do anything differently in the cloud? We must treat each region as we treat our own data centers and think about Geo-HA.

The rest of this post is about taking high availably to the next level by deploying a Geo-HA Service Fabric cluster. Continue Reading…


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Azure SQL Database doesn’t support Heap Tables. Thus creating tables in Azure SQL Database requires Clustered Indexes. By default Azure SQL Database creates the Clustered Index on the table’s Primary Key.

CREATE TABLE [dbo].[TestTable] (
    [Id]        UNIQUEIDENTIFIER NOT NULL,
    [FirstName] NVARCHAR (10)    NOT NULL,
    [LastName]  NVARCHAR (10)    NOT NULL,
    [Type]      INT              NOT NULL,
    [Timestamp] ROWVERSION       NOT NULL,
    PRIMARY KEY (Id));

Primary Keys often make good Clustered Indexes because data is regularly accessed through them. But there are times when the Primary Key just doesn’t cut it. Fortunately, you can define a Clustered Index on other columns.

CREATE TABLE [dbo].[TestTable] (
    [Id]        UNIQUEIDENTIFIER NOT NULL,
    [FirstName] NVARCHAR (10)    NOT NULL,
    [LastName]  NVARCHAR (10)    NOT NULL,
    [Type]      INT              NOT NULL,
    [Timestamp] ROWVERSION       NOT NULL,
    PRIMARY KEY NONCLUSTERED (Id));
CREATE CLUSTERED INDEX IX_TestTable on TestTable (FirstName, LastName);

Changing a clustered index in Azure SQL Database is possible but it comes with some constraints. Failing to satisfy the existing  constraints enforce by the index will result in “The new index definition does not match the constraint being enforced by the existing index”.

CREATE CLUSTERED INDEX IX_TestTable
ON TestTable ([Type],[FirstName],[LastName])
WITH (DROP_EXISTING=ON)
ON [Primary]

Continue Reading…