address Max's comments

README.md

@@ -526,7 +526,7 @@ We cover security basics first, since configuring user accounts is something you
 -	At the beginning, IAM policy may be very simple, but for large systems, it will grow in complexity, and need to be managed with care.
 	-	🔹Make sure one person (perhaps with a backup) in your organization is formally assigned ownership of managing IAM policies, make sure every administrator works with that person to have changes reviewed. This goes a long way to avoiding accidental and serious misconfigurations.
 -	It is best to give each user or service the minimum privileges needed to perform their duties. This is the [principle of least privilege](https://en.wikipedia.org/wiki/Principle_of_least_privilege), one of the foundations of good security. Organize all IAM users and groups according to levels of access they need.
--	IAM has the permission hierarchy of 
+-	IAM has the permission hierarchy of:
 	1. Explicit deny: The most restrictive policy wins.
 	2. Explicit allow: Access permissions to any resource has to be explicitly given.
 	3. Implicit deny: All permissions are implicitly denied by default.
@@ -663,7 +663,7 @@ S3
 -	If you are primarily using a VPC, consider setting up a [VPC Endpoint](http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-endpoints.html) for S3 in order to allow your VPC-hosted resources to easily access it without the need for extra network configuration or hops.
 -	**Cross-region replication:** S3 has [a feature](https://docs.aws.amazon.com/AmazonS3/latest/dev/crr.html) for replicating a bucket between one region and a another. Note that S3 is already highly replicated within one region, so usually this isn’t necessary for durability, but it could be useful for compliance (geographically distributed data storage), lower latency, or as a strategy to reduce region-to-region bandwidth costs by mirroring heavily used data in a second region.
 -	**IPv4 vs IPv6:** For a long time S3 only supported IPv4 at the default endpoint `https://BUCKET.s3.amazonaws.com`. However, [as of Aug 11, 2016](https://aws.amazon.com/blogs/aws/now-available-ipv6-support-for-amazon-s3/) it now supports both IPv4 & IPv6! To use both, you have to [enable dualstack](http://docs.aws.amazon.com/AmazonS3/latest/dev/dual-stack-endpoints.html) either in your preferred API client or by directly using this url scheme `https://BUCKET.s3.dualstack.REGION.amazonaws.com`.
--	**S3 Event Notifications:** S3 can be configured to send an SNS notification, SQS message, or AWS Lambda function on bucket events. [doc](http://docs.aws.amazon.com/AmazonS3/latest/dev/NotificationHowTo.html)
+-	**S3 Event Notifications:** S3 can be configured to send an [SNS notification](https://aws.amazon.com/blogs/aws/introducing-the-amazon-simple-notification-service/), [SQS message](http://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/Welcome.html), or [AWS Lambda function](http://docs.aws.amazon.com/lambda/latest/dg/welcome.html) on bucket events. [doc](http://docs.aws.amazon.com/AmazonS3/latest/dev/NotificationHowTo.html)
 ### S3 Gotchas and Limitations
 
 -	🔸For many years, there was a notorious [**100-bucket limit**](https://docs.aws.amazon.com/general/latest/gr/aws_service_limits.html#limits_s3) per account, which could not be raised and caused many companies significant pain. As of 2015, you can [request increases](https://aws.amazon.com/about-aws/whats-new/2015/08/amazon-s3-introduces-new-usability-enhancements/). You can ask to increase the limit, but it will still be capped (generally below ~1000 per account).
@@ -672,7 +672,7 @@ S3
 -	🔸After uploading, any change that you make to the object causes a full rewrite of the object, so avoid appending-like behavior with regular files.
 -	🔸Eventual data consistency, as discussed above, can be surprising sometimes. If S3 suffers from internal replication issues, an object may be visible from a subset of the machines, depending on which S3 endpoint they hit. Those usually resolve within seconds; however, we’ve seen isolated cases when the issue lingered for 20-30 hours.
 -	🔸**MD5s and multi-part uploads:** In S3, the [ETag header in S3](http://docs.aws.amazon.com/AmazonS3/latest/API/RESTCommonResponseHeaders.html) is a hash on the object. And in many cases, it is the MD5 hash. However, this [is not the case in general](http://stackoverflow.com/questions/12186993/what-is-the-algorithm-to-compute-the-amazon-s3-etag-for-a-file-larger-than-5gb) when you use multi-part uploads. One workaround is to compute MD5s yourself and put them in a custom header (such as is done by [s4cmd](https://github.com/bloomreach/s4cmd)).
--	🔸**US Standard region:** Most S3 endpoints match the region they’re in, with the exception of the us-east-1 region, which is called 'us-standard' in S3 terminology. This region is also the only region that is replicated across coasts. As a result, latency varies more in this region than in others. You can minimize latency from us-east-1 by using *[s3-external-1.amazonaws.com](http://s3-external-1.amazonaws.com/)*.  
+-	🔸**US Standard region:** Most S3 endpoints match the region they’re in, with the exception of the us-east-1 region, which is called 'us-standard' in S3 terminology. This region is also the only region that is replicated across coasts. As a result, latency varies more in this region than in others. You can minimize latency from us-east-1 by using *[s3-external-1.amazonaws.com](http://s3-external-1.amazonaws.com/)*.
 
 ### Storage Durability, Availability, and Price
 
@@ -1233,7 +1233,7 @@ Redshift
 -	[Amazon Redshift Utils](https://github.com/awslabs/amazon-redshift-utils) contains useful utilities, scripts and views to simplify Redshift ops.
 -	[VACUUM](http://docs.aws.amazon.com/redshift/latest/dg/t_Reclaiming_storage_space202.html) regularly following a significant number of deletes or updates to reclaim space and improve query performance.
 -	Redshift provides various [column compression](http://docs.aws.amazon.com/redshift/latest/dg/t_Compressing_data_on_disk.html) options to optimize the stored data size. AWS strongly encourages users to use [automatic compression](http://docs.aws.amazon.com/redshift/latest/dg/c_Loading_tables_auto_compress.html) at the COPY stage, when Redshift uses a sample of the data being ingested to analyze the column compression options. However, automatic compression can only be applied to an empty table with no data. Therefore, make sure the initial load batch is big enough to provide Redshift with a representative sample of the data (the default sample size is 100000 rows).
--	Redshift uses columnar storage, hence it does not have indexing capabilities. You can, however, use distribution key [distkey](http://docs.aws.amazon.com/redshift/latest/dg/c_best-practices-best-dist-key.html)and sort key [sortkey](docs.aws.amazon.com/redshift/latest/dg/c_best-practices-sort-key.html)to improve performance. Redshift has two type of sort keys: compounding sort key and interleaved sort key
+-	Redshift uses columnar storage, hence it does not have indexing capabilities. You can, however, use distribution key [distkey](http://docs.aws.amazon.com/redshift/latest/dg/c_best-practices-best-dist-key.html) and sort key [sortkey](docs.aws.amazon.com/redshift/latest/dg/c_best-practices-sort-key.html) to improve performance. Redshift has two type of sort keys: compounding sort key and interleaved sort key.
 -	A compound sort key is made up of all columns listed in the sort key definition. It is most useful when you have queries with operations using prefix of the sortkey.
 -	An interleaved sort key on the other hand gives equal weight to each column or a subset of columns in the sort key. So if you don't know ahead of time which column you want to choose for sorting and filtering, this is a much better choice than the compound key.[Here](https://aws.amazon.com/blogs/aws/quickly-filter-data-in-amazon-redshift-using-interleaved-sorting/) is an example using interleaved sort key.
 
@@ -1250,7 +1250,7 @@ Redshift
 -	❗ Never resize a live cluster. The resize operation takes hours depending on the dataset size. In rare cases, the operation may also get stuck and you'll end up having a non-functional cluster. The safer approach is to create a new cluster from a snapshot, resize the new cluster and shut down the old one.
 -	Redshift has reserved keywords which are not present in Postgres (see full list [here](https://docs.aws.amazon.com/redshift/latest/dg/r_pg_keywords.html)). Watch out for DELTA ([Delta Encodings](https://docs.aws.amazon.com/redshift/latest/dg/c_Delta_encoding.html)).
 -	Redshift does not support many Postgres functions, most notably several date/time-related and aggregation functions. See the [full list here](https://docs.aws.amazon.com/redshift/latest/dg/c_unsupported-postgresql-functions.html).
--	Compression on sortkey can result in significant performance impact. So if your Redshift queries involving sort key(s) are slow, you might want to consider removing compression on your sort key(s)[Data Compression](https://aws.amazon.com/blogs/big-data/optimizing-for-star-schemas-and-interleaved-sorting-on-amazon-redshift/).
+-	Compression on sortkey can result in significant performance impact. So if your Redshift queries involving sort key(s) are slow, you might want to consider removing compression on a sortkey. See [Data Compression](https://aws.amazon.com/blogs/big-data/optimizing-for-star-schemas-and-interleaved-sorting-on-amazon-redshift/).
 
 EMR
 ---
@@ -1269,9 +1269,8 @@ EMR
 -	EMR relies on many versions of Hadoop and other supporting software. Be sure to check [which versions are in use](https://docs.aws.amazon.com/ElasticMapReduce/latest/ReleaseGuide/emr-release-components.html).
 -	⏱Off-the-shelf EMR and Hadoop can have significant overhead when compared with efficient processing on a single machine. If your data is small and performance matters, you may wish to consider alternatives, as [this post](http://aadrake.com/command-line-tools-can-be-235x-faster-than-your-hadoop-cluster.html) illustrates.
 -	Python programmers may want to take a look at Yelp’s [mrjob](https://github.com/Yelp/mrjob).
--	Larger instances don't neccessarily cost more in spot market. So you should look at different options and determine which instances you should bid for your jobs.
--	Since you are billed at hour granularity, it is usually beneficial to increase the number of instances and/or the type of intances to keep the time of running your EMR jobs under one hour.
--	Bootstrap Actions [doc](http://docs.aws.amazon.com/ElasticMapReduce/latest/DeveloperGuide/emr-plan-bootstrap.html) are used to configure number of mappers/reducers/amount of memory of your EMR jobs. They are also used for installing extra softwares on for your cluster before your EMR jobs are running.
+-	Larger instances don't neccessarily cost more in spot market. Therefore, you should look at different options and determine which instances you should bid for your jobs. See [Bid Advisor](https://aws.amazon.com/ec2/spot/bid-advisor/).
+-	Since EMR jobs are billed at hour granularity, it is usually beneficial to increase the number of instances and/or the type of instances to keep the time of running your EMR jobs under one hour.
 -	It takes time to tune performance of EMR jobs, which is why third-party services such as [Qubole’s data service](https://www.qubole.com/mapreduce-as-a-service/) are gaining popularity as ways to improve performance or reduce costs.
 
 ### EMR Gotchas and Limitations